ath9k: fix ibss beacon next tbtt

Sync-up ibss beacon timer with the beacon frame's timestamp. When the
node acts as joiner, it has to sync with the received beacon timestamp
instead of reading tsf from hw. As the hw tsf wont wont be update till
bssid is configured. This patch programs hw tsf with the received beacon
timestamp if beacon timers are yet to be configured.

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