umbim: fix return value of proto_mbim_setup()

The variable name appears to be mistyped.

Suggested-by: Howard Chu <[email protected]>
Signed-off-by: Adrian Schmutzler <[email protected]>

package/network/utils/umbim/files/lib/netifd/proto/mbim.sh

@@ -155,7 +155,7 @@ proto_mbim_setup() {
 		sleep 15
 	}
 
-	return $rt
+	return $ret
 }
 
 proto_mbim_teardown() {