net: dsa: mt7530: move autoneg handling to PCS validation

Move the autoneg bit handling to the PCS validation, which allows us to
get rid of mt753x_phylink_validate() and rely on the default
phylink_generic_validate() implementation for the MAC side.

Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>