ath9k: Process beacons properly

When the current operating channel context has
been marked as ATH_CHANCTX_STATE_FORCE_ACTIVE,
do not process beacons that might be received,
since we have to wait for the station to become
authorized.

Also, since the cached TSF value will be zero
initially do not rearm the timer in this
case when a beacon is received, since it results
in spurious values.

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