ar9170: fix hang on stop

This patch fixes a locking problem which freezes the network core.

The deadlock goes as follows:
- ar9170_op_stop - is called
1. change the state to IDLE
2. > take the MUTEX <
3. cancel_SYNC all pending work, which means
"block until a work_struct's callback has terminated"
=> if filter_config_work was queued it tries to get the MUTEX,
before checking the device state...

Signed-off-by: Christian Lamparter <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/ar9170/main.c

···
 	if (IS_STARTED(ar))
 		ar->state = AR9170_IDLE;
 
-	mutex_lock(&ar->mutex);
+	flush_workqueue(ar->hw->workqueue);
 
+	mutex_lock(&ar->mutex);
 	cancel_delayed_work_sync(&ar->tx_status_janitor);
 	cancel_work_sync(&ar->filter_config_work);
 	cancel_work_sync(&ar->beacon_work);
···
 					 filter_config_work);
 	int err;
 
-	mutex_lock(&ar->mutex);
 	if (unlikely(!IS_STARTED(ar)))
-		goto unlock;
+		return ;
 
+	mutex_lock(&ar->mutex);
 	if (ar->filter_changed & AR9170_FILTER_CHANGED_PROMISC) {
 		err = ar9170_set_operating_mode(ar);
 		if (err)