ar76c50x-usb: cancel promisc work during mac80211 stop

We weren't ever cancelling this.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/at76c50x-usb.c

@@ -1773,6 +1773,8 @@ static void at76_mac80211_stop(struct ieee80211_hw *hw)
 
 	at76_dbg(DBG_MAC80211, "%s()", __func__);
 
+	cancel_work_sync(&priv->work_set_promisc);
+
 	mutex_lock(&priv->mtx);
 
 	if (!priv->device_unplugged) {