mac802154/iface: remove superfluous WARN_ON call in slave_open()

This call was used before we aligned our code with the wireless code base. We
are wanted to handle this in the err: code path. Which would actually not work
because the WARN_ON() macro would reset the res value to 0 and thus we would
never hit err:. Removing it makes the code do what we actually intend.
Signed-off-by: NStefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/mac802154/iface.c

@@ -147,7 +147,6 @@ static int mac802154_slave_open(struct net_device *dev)
 
 	if (!local->open_count) {
 		res = drv_start(local);
-		WARN_ON(res);
 		if (res)
 			goto err;
 	}