net: mvneta: convert to pcs_validate() and phylink_generic_validate()

Convert mvneta to validate the autoneg state for 1000base-X in the
pcs_validate() operation, rather than the MAC validate() operation.
This allows us to switch the MAC validate() to use
phylink_generic_validate().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>