ath9k: Remove unnecessary tbtt assignment

The next_tbtt variable in the scheduler is needed only
for GO/AP mode operation and is always read from the
NEXT_TBTT_TIMER in the HW. There is no need to store
the timestamp for received beacons.

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