Bluetooth: Fix H4 crash from incoming UART packets

Bluetooth: Fix H4 crash from incoming UART packets

This patch adds a check HCI_UART_REGISTERED before reading UART data in
the HCI UART H4 driver. UART data could arrive when inside the
hci_uart_tty_ioctl function after calling test_and_set_bit for
HCI_UART_PROTO_SET but before the hci_uart_set_proto function has
returned.

Backtrace:
[<c05f27ec>] (hci_recv_stream_fragment+0x0/0x74) from [<c04126f4>] (h4_recv+0x18/0x40)
 r7:eb1d4d1c r6:eb7683b0 r5:eae8e800 r4:0000000c
[<c04126dc>] (h4_recv+0x0/0x40) from [<c0411870>] (hci_uart_tty_receive+0x6c/0x94)
 r5:eae8e800 r4:eb768380
[<c0411804>] (hci_uart_tty_receive+0x0/0x94) from [<c027be88>] (flush_to_ldisc+0x16c/0x17c)
 r6:eae8e8d8 r5:eae8e800 r4:eae8e8c8
[<c027bd1c>] (flush_to_ldisc+0x0/0x17c) from [<c0050ae8>] (process_one_work+0x144/0x4d4)
[<c00509a4>] (process_one_work+0x0/0x4d4) from [<c0051208>] (worker_thread+0x180/0x370)
[<c0051088>] (worker_thread+0x0/0x370) from [<c005617c>] (kthread+0x90/0x9c)
[<c00560ec>] (kthread+0x0/0x9c) from [<c003a3a0>] (do_exit+0x0/0x7ec)

Signed-off-by: Chan-yeol Park <chanyeol.park@samsung.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>