net: ethernet: mvneta: Fix Serdes configuration for SoCs without comphy

The MVNETA_SERDES_CFG register is only available on older SoCs like the
Armada XP. On newer SoCs like the Armada 38x the fields are moved to
comphy. This patch moves the writes to this register next to the comphy
initialization, so that depending on the SoC either comphy or
MVNETA_SERDES_CFG is configured.
With this we no longer write to the MVNETA_SERDES_CFG on SoCs where it
doesn't exist.

Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>