Bluetooth: fix error return code in rfcomm_add_listener()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

net/bluetooth/rfcomm/core.c

@@ -2004,8 +2004,10 @@ static int rfcomm_add_listener(bdaddr_t *ba)
 
 	/* Add listening session */
 	s = rfcomm_session_add(sock, BT_LISTEN);
-	if (!s)
+	if (!s) {
+		err = -ENOMEM;
 		goto failed;
+	}
 
 	return 0;
 failed: