ath9k: Disable SWBA interrupt on remove_interface

while removing beaconing mode interface, SWBA interrupt
was never disabled when there are no other beaconing interfaces.

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