net: sparx5: clean up sparx5_phylink_validate()

sparx5_phylink_validate() no longer needs to check for
PHY_INTERFACE_MODE_NA as phylink will walk the supported interface
types to discover the link mode capabilities. Neither is it necessary
to check the device capabilities as we will not be called for
unsupported interface modes. Remove these checks.

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