mac80211: fix beacon loss detection after scan

Currently beacon loss detection triggers after a scan. A probe request
is sent and a message like this is printed to the log:

wlan0: beacon loss from AP 00:12:17:e7:98:de - sending probe request

But in fact there is no beacon loss, the beacons are just not received
because of the ongoing scan. Fix it by updating last_beacon after
the scan has finished.
Reported-by: Jaswinder Singh Rajput <jaswinder@kernel.org>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/mlme.c

@@ -1913,9 +1913,17 @@ static void ieee80211_sta_work(struct work_struct *work)
 
 static void ieee80211_restart_sta_timer(struct ieee80211_sub_if_data *sdata)
 {
-	if (sdata->vif.type == NL80211_IFTYPE_STATION)
+	if (sdata->vif.type == NL80211_IFTYPE_STATION) {
+		/*
+		 * Need to update last_beacon to avoid beacon loss
+		 * test to trigger.
+		 */
+		sdata->u.mgd.last_beacon = jiffies;
+
+
 		queue_work(sdata->local->hw.workqueue,
 			   &sdata->u.mgd.work);
+	}
 }
 
 /* interface setup */