net: dsa: mv88e6xxx: convert to phylink_generic_validate()

Now that the mv88e6xxx chip drivers are supplying the supported
interfaces and MAC capabilities, switch the driver to use the generic
phylink validation implementation by removing our own validation
implementations. This causes DSA to call phylink_generic_validate()
on our behalf.

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