net: lwip: do not return CMD_RET_USAGE if no interface

If the dns command cannot find a network interface, we should return
CMD_RETFAIURE and not -1 (CMD_RET_USAGE).

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

net/lwip/dns.c

@@ -56,7 +56,7 @@ static int dns_loop(struct udevice *udev, const char *name, const char *var)
 
 	netif = net_lwip_new_netif(udev);
 	if (!netif)
-		return -1;
+		return CMD_RET_FAILURE;
 
 	dns_init();