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>
diff --git a/net/mac802154/iface.c b/net/mac802154/iface.c
index d804337..692731d 100644
--- a/net/mac802154/iface.c
+++ b/net/mac802154/iface.c
@@ -308,6 +308,7 @@
 	clear_bit(SDATA_STATE_RUNNING, &sdata->state);
 
 	if (!local->open_count) {
+		flush_workqueue(local->workqueue);
 		hrtimer_cancel(&local->ifs_timer);
 		drv_stop(local);
 	}