ath9k_htc: Queue WMI events

Use a queue to handle WMI events and schedule a tasklet
to process the events. This fixes the race between the
WMI event ISR and the SWBA tasklet when the arrival of
WMI events in quick succession could overwrite the SWBA
data before the tasklet from a previous iteration could
have been scheduled. Also, drain the WMI queue properly.

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