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>
1 file changed