ath9k: Adjust tbtt delta properly

In a GO/STA setup, when we switch to the STA context,
the channel context timer is scheduled with a period of
half the beacon interval. If a beacon is received in
this duration, the timer is adjusted to accommodate
TSF sync done by the HW.

But, if the actual channel switch is delayed for some
reason, we end up rearming the timer every time a new
beacon is received. Avoid this by doing the adjustment
only once.

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