ath9k: fix beacon restart on channel change

Restart the beacon timers only if the beacon
was already configured. Otherwise beacons timers
are restarted unnecessarily in unassociated state too.

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