mac802154: iface: flush workqueue before stop

This patch flushs the workqueue which is currently used for xmit_sync
callback before calling stop driver-ops. Flush the queue will ensure all
pending tx frames are transmitted.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/mac802154/iface.c

@@ -308,6 +308,7 @@ static int mac802154_slave_close(struct net_device *dev)
 	clear_bit(SDATA_STATE_RUNNING, &sdata->state);
 
 	if (!local->open_count) {
+		flush_workqueue(local->workqueue);
 		hrtimer_cancel(&local->ifs_timer);
 		drv_stop(local);
 	}