ath9k: disable beaconing before stopping beacon queue

Beaconing should be disabled before stopping beacon queue.
Not doing so could queue up beacons in hw that causes
failure to stop Tx DMA, due to pending frames in hw
and also unnecessary beacon tasklet schedule.

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