net: mvneta: use phylink_generic_validate()

Convert mvneta to use phylink_generic_validate() for the bulk of its
validate() implementation. This network adapter has a restriction
that for 802.3z links, autonegotiation must be enabled.

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