wl12xx: implement cancel_hw_scan() callback

wl12xx: implement cancel_hw_scan() callback

After implementing cancel_hw_scan(), there is no more need to
flush scan_complete_work on suspend.

scan_complete_work enqueues a new scan_work, that might lead
to the following warning (if the device was already suspended):

WARNING: at drivers/mmc/core/core.c:169 mmc_wait_for_req+0x1ac/0x2c8()
Backtrace:
[<c004b50c>] (dump_backtrace+0x0/0x110)
[<c04fda50>] (dump_stack+0x0/0x1c)
[<c008f700>] (warn_slowpath_common+0x0/0x6c)
[<c008f76c>] (warn_slowpath_null+0x0/0x2c)
[<c034b058>] (mmc_wait_for_req+0x0/0x2c8)
[<c034f804>] (mmc_io_rw_extended+0x0/0x1e4)
[<c0350b5c>] (sdio_io_rw_ext_helper+0x0/0x1ac)
[<c0350e00>] (sdio_memcpy_fromio+0x0/0x2c)
[<bf194464>] (wl1271_sdio_raw_read+0x0/0x1bc [wl12xx_sdio])
[<bf15ac14>] (wl1271_cmd_wait_for_event_or_timeout+0x0/0x19c [wl12xx])
[<bf15adb0>] (wl1271_cmd_wait_for_event+0x0/0x30 [wl12xx])
[<bf15b7dc>] (wl1271_cmd_disconnect+0x0/0x14c [wl12xx])
[<bf158fd0>] (wl1271_unjoin+0x0/0x48 [wl12xx])
[<bf159070>] (wl1271_op_config+0x0/0x490 [wl12xx])
[<bf105084>] (ieee80211_hw_config+0x0/0x14c [mac80211])
[<bf113120>] (ieee80211_recalc_idle+0x0/0x4c [mac80211])
[<bf109a70>] (__ieee80211_scan_completed+0x0/0x1ac [mac80211])
[<bf109fd4>] (ieee80211_scan_work+0x0/0x484 [mac80211])
[<c00a197c>] (worker_thread+0x0/0x1e8)
[<c00a5874>] (kthread+0x0/0x90)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>