net: mvneta: remove interface checks in mvneta_validate()

As phylink checks the interface mode against the supported_interfaces
bitmap, we no longer need to validate the interface mode in the
validation function. Remove this to simplify it.

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