In mvneta_config_interface() the RGMII modes are catched by the default
case which is an error return. The RGMII modes are valid modes for the
driver, so instead of returning an error add a break statement to return
successfully.

This avoids this warning for non comphy SoCs which use RGMII, like
SolidRun Clearfog:

WARNING: CPU: 0 PID: 268 at drivers/net/ethernet/marvell/mvneta.c:3512 mvneta_start_dev+0x220/0x23c

Fixes: b4748553 ("net: ethernet: mvneta: Fix Serdes configuration for SoCs without comphy")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>