ath9k: Use TSFOOR interrupt to trigger TSF sync with next Beacon

If the chip complains about TSF sync, make sure we remain awake to
sync with the next Beacon frame. In theory, this should not be needed
since we are currently trying to receive all Beacon frames, anyway,
better have this code ready should we ever change that.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>