ath9k: Process BB watchdog events in the tasklet

Move the BB processing code to the tasklet and avoid
doing it in the ISR, there is no real benefit and this
makes the ISR less heavy.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>