ath9k: Resync beacons properly

After a chip reset, the beacon timers have to re-programmed
correctly for a station in associated state. Use the PS flags
to ensure that this is done after a TSF sync happens, otherwise
the driver ends up using incorrect values for TBTT/DTIM in
powersave mode.

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