rt2x00: check if device is still available on rt2x00mac_flush()

rt2x00: check if device is still available on rt2x00mac_flush()

Fix random kernel panic with below messages when remove dongle.

[ 2212.355447] BUG: unable to handle kernel NULL pointer dereference at 0000000000000250
[ 2212.355527] IP: [<ffffffffa02667f2>] rt2x00usb_kick_tx_entry+0x12/0x160 [rt2x00usb]
[ 2212.355599] PGD 0
[ 2212.355626] Oops: 0000 [#1] SMP
[ 2212.355664] Modules linked in: rt2800usb rt2x00usb rt2800lib crc_ccitt rt2x00lib mac80211 cfg80211 tun arc4 fuse rfcomm bnep snd_hda_codec_realtek snd_hda_intel snd_hda_codec btusb uvcvideo bluetooth snd_hwdep x86_pkg_temp_thermal snd_seq coretemp aesni_intel aes_x86_64 snd_seq_device glue_helper snd_pcm ablk_helper videobuf2_vmalloc sdhci_pci videobuf2_memops videobuf2_core sdhci videodev mmc_core serio_raw snd_page_alloc microcode i2c_i801 snd_timer hid_multitouch thinkpad_acpi lpc_ich mfd_core snd tpm_tis wmi tpm tpm_bios soundcore acpi_cpufreq i915 i2c_algo_bit drm_kms_helper drm i2c_core video [last unloaded: cfg80211]
[ 2212.356224] CPU: 0 PID: 34 Comm: khubd Not tainted 3.12.0-rc3-wl+ #3
[ 2212.356268] Hardware name: LENOVO 3444CUU/3444CUU, BIOS G6ET93WW (2.53 ) 02/04/2013
[ 2212.356319] task: ffff880212f687c0 ti: ffff880212f66000 task.ti: ffff880212f66000
[ 2212.356392] RIP: 0010:[<ffffffffa02667f2>]  [<ffffffffa02667f2>] rt2x00usb_kick_tx_entry+0x12/0x160 [rt2x00usb]
[ 2212.356481] RSP: 0018:ffff880212f67750  EFLAGS: 00010202
[ 2212.356519] RAX: 000000000000000c RBX: 000000000000000c RCX: 0000000000000293
[ 2212.356568] RDX: ffff8801f4dc219a RSI: 0000000000000000 RDI: 0000000000000240
[ 2212.356617] RBP: ffff880212f67778 R08: ffffffffa02667e0 R09: 0000000000000002
[ 2212.356665] R10: 0001f95254ab4b40 R11: ffff880212f675be R12: ffff8801f4dc2150
[ 2212.356712] R13: 0000000000000000 R14: ffffffffa02667e0 R15: 000000000000000d
[ 2212.356761] FS:  0000000000000000(0000) GS:ffff88021e200000(0000) knlGS:0000000000000000
[ 2212.356813] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2212.356852] CR2: 0000000000000250 CR3: 0000000001a0c000 CR4: 00000000001407f0
[ 2212.356899] Stack:
[ 2212.356917]  000000000000000c ffff8801f4dc2150 0000000000000000 ffffffffa02667e0
[ 2212.356980]  000000000000000d ffff880212f677b8 ffffffffa03a31ad ffff8801f4dc219a
[ 2212.357038]  ffff8801f4dc2150 0000000000000000 ffff8800b93217a0 ffff8801f49bc800
[ 2212.357099] Call Trace:
[ 2212.357122]  [<ffffffffa02667e0>] ? rt2x00usb_interrupt_txdone+0x90/0x90 [rt2x00usb]
[ 2212.357174]  [<ffffffffa03a31ad>] rt2x00queue_for_each_entry+0xed/0x170 [rt2x00lib]
[ 2212.357244]  [<ffffffffa026701c>] rt2x00usb_kick_queue+0x5c/0x60 [rt2x00usb]
[ 2212.357314]  [<ffffffffa03a3682>] rt2x00queue_flush_queue+0x62/0xa0 [rt2x00lib]
[ 2212.357386]  [<ffffffffa03a2930>] rt2x00mac_flush+0x30/0x70 [rt2x00lib]
[ 2212.357470]  [<ffffffffa04edded>] ieee80211_flush_queues+0xbd/0x140 [mac80211]
[ 2212.357555]  [<ffffffffa0502e52>] ieee80211_set_disassoc+0x2d2/0x3d0 [mac80211]
[ 2212.357645]  [<ffffffffa0506da3>] ieee80211_mgd_deauth+0x1d3/0x240 [mac80211]
[ 2212.357718]  [<ffffffff8108b17c>] ? try_to_wake_up+0xec/0x290
[ 2212.357788]  [<ffffffffa04dbd18>] ieee80211_deauth+0x18/0x20 [mac80211]
[ 2212.357872]  [<ffffffffa0418ddc>] cfg80211_mlme_deauth+0x9c/0x140 [cfg80211]
[ 2212.357913]  [<ffffffffa041907c>] cfg80211_mlme_down+0x5c/0x60 [cfg80211]
[ 2212.357962]  [<ffffffffa041cd18>] cfg80211_disconnect+0x188/0x1a0 [cfg80211]
[ 2212.358014]  [<ffffffffa04013bc>] ? __cfg80211_stop_sched_scan+0x1c/0x130 [cfg80211]
[ 2212.358067]  [<ffffffffa03f8954>] cfg80211_leave+0xc4/0xe0 [cfg80211]
[ 2212.358124]  [<ffffffffa03f8d1b>] cfg80211_netdev_notifier_call+0x3ab/0x5e0 [cfg80211]
[ 2212.358177]  [<ffffffff815140f8>] ? inetdev_event+0x38/0x510
[ 2212.358217]  [<ffffffff81085a94>] ? __wake_up+0x44/0x50
[ 2212.358254]  [<ffffffff8155995c>] notifier_call_chain+0x4c/0x70
[ 2212.358293]  [<ffffffff81081156>] raw_notifier_call_chain+0x16/0x20
[ 2212.358361]  [<ffffffff814b6dd5>] call_netdevice_notifiers_info+0x35/0x60
[ 2212.358429]  [<ffffffff814b6ec9>] __dev_close_many+0x49/0xd0
[ 2212.358487]  [<ffffffff814b7028>] dev_close_many+0x88/0x100
[ 2212.358546]  [<ffffffff814b8150>] rollback_registered_many+0xb0/0x220
[ 2212.358612]  [<ffffffff814b8319>] unregister_netdevice_many+0x19/0x60
[ 2212.358694]  [<ffffffffa04d8eb2>] ieee80211_remove_interfaces+0x112/0x190 [mac80211]
[ 2212.358791]  [<ffffffffa04c585f>] ieee80211_unregister_hw+0x4f/0x100 [mac80211]
[ 2212.361994]  [<ffffffffa03a1221>] rt2x00lib_remove_dev+0x161/0x1a0 [rt2x00lib]
[ 2212.365240]  [<ffffffffa0266e2e>] rt2x00usb_disconnect+0x2e/0x70 [rt2x00usb]
[ 2212.368470]  [<ffffffff81419ce4>] usb_unbind_interface+0x64/0x1c0
[ 2212.371734]  [<ffffffff813b446f>] __device_release_driver+0x7f/0xf0
[ 2212.374999]  [<ffffffff813b4503>] device_release_driver+0x23/0x30
[ 2212.378131]  [<ffffffff813b3c98>] bus_remove_device+0x108/0x180
[ 2212.381358]  [<ffffffff813b0565>] device_del+0x135/0x1d0
[ 2212.384454]  [<ffffffff81417760>] usb_disable_device+0xb0/0x270
[ 2212.387451]  [<ffffffff8140d9cd>] usb_disconnect+0xad/0x1d0
[ 2212.390294]  [<ffffffff8140f6cd>] hub_thread+0x63d/0x1660
[ 2212.393034]  [<ffffffff8107c860>] ? wake_up_atomic_t+0x30/0x30
[ 2212.395728]  [<ffffffff8140f090>] ? hub_port_debounce+0x130/0x130
[ 2212.398412]  [<ffffffff8107baa0>] kthread+0xc0/0xd0
[ 2212.401058]  [<ffffffff8107b9e0>] ? insert_kthread_work+0x40/0x40
[ 2212.403639]  [<ffffffff8155de3c>] ret_from_fork+0x7c/0xb0
[ 2212.406193]  [<ffffffff8107b9e0>] ? insert_kthread_work+0x40/0x40
[ 2212.408732] Code: 24 58 08 00 00 bf 80 00 00 00 e8 3a c3 e0 e0 5b 41 5c 5d c3 0f 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 53 <48> 8b 47 10 48 89 fb 4c 8b 6f 28 4c 8b 20 49 8b 04 24 4c 8b 30
[ 2212.414671] RIP  [<ffffffffa02667f2>] rt2x00usb_kick_tx_entry+0x12/0x160 [rt2x00usb]
[ 2212.417646]  RSP <ffff880212f67750>
[ 2212.420547] CR2: 0000000000000250
[ 2212.441024] ---[ end trace 5442918f33832bce ]---

Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>