nl80211: Clear beacon_set when deleting a beacon from deinit_ap()

When a beacon was deleted from the kernel in
wpa_driver_nl80211_deinit_ap(), bss->beacon_set was not cleared so
restarting the AP again was not possible.

Fix this by clearing the variable once the beacon was deleted.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>

src/drivers/driver_nl80211.c

@@ -11132,6 +11132,7 @@ static int wpa_driver_nl80211_deinit_ap(void *priv)
 	if (!is_ap_interface(drv->nlmode))
 		return -1;
 	wpa_driver_nl80211_del_beacon(drv);
+	bss->beacon_set = 0;
 
 	/*
 	 * If the P2P GO interface was dynamically added, then it is