ath9k: Update Beacon timers based on timestamp from the AP

Some APs seem to drift away from the expected TBTT (timestamp %
beacon_int_in_usec differs quite a bit from zero) which can result in
us waking up way too early to receive a Beacon frame. In order to work
around this, re-configure the Beacon timers after having received a
Beacon frame from the AP (i.e., when we know the offset between the
expected TBTT and the actual time the AP is sending out the Beacon
frame).

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

drivers/net/wireless/ath/ath9k/ath9k.h

@@ -518,6 +518,7 @@ struct ath_rfkill {
 #define SC_OP_WAIT_FOR_CAB      BIT(16)
 #define SC_OP_WAIT_FOR_PSPOLL_DATA BIT(17)
 #define SC_OP_WAIT_FOR_TX_ACK   BIT(18)
+#define SC_OP_BEACON_SYNC       BIT(19)
 
 struct ath_bus_ops {
 	void		(*read_cachesize)(struct ath_softc *sc, int *csz);