ath9k: move ps helpers onto core driver when reseting tsf

When the TSF is reset power save state is disabled and
then restored. The helpers to disable power save and restore
it use a lock provided by the driver core. Move the callers
of the helpers outside of the hw code.

We reset the TSF when mac80211 tells us and on the beacon.c
helper ath9k_hw_beaconinit() when it is made explicitly required.
Add a helper on beacon.c which will deal with ps awake/restore
if we need to reset the TSF upon ath9k_hw_beaconinit().

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>