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>
diff --git a/drivers/net/wireless/ath/ath9k/main.c b/drivers/net/wireless/ath/ath9k/main.c
index 5759c94..61da08a 100644
--- a/drivers/net/wireless/ath/ath9k/main.c
+++ b/drivers/net/wireless/ath/ath9k/main.c
@@ -487,7 +487,7 @@
 		 * the next Beacon.
 		 */
 		DPRINTF(sc, ATH_DBG_PS, "TSFOOR - Sync with next Beacon\n");
-		sc->sc_flags |= SC_OP_WAIT_FOR_BEACON;
+		sc->sc_flags |= SC_OP_WAIT_FOR_BEACON | SC_OP_BEACON_SYNC;
 	}
 
 	/* re-enable hardware interrupt */
@@ -914,6 +914,13 @@
 		if (avp->av_opmode == NL80211_IFTYPE_STATION) {
 			sc->curaid = bss_conf->aid;
 			ath9k_hw_write_associd(sc);
+
+			/*
+			 * Request a re-configuration of Beacon related timers
+			 * on the receipt of the first Beacon frame (i.e.,
+			 * after time sync with the AP).
+			 */
+			sc->sc_flags |= SC_OP_BEACON_SYNC;
 		}
 
 		/* Configure the beacon */