net: dsa: mv88e6xxx: Fix phylink_validate for Topaz family

The Topaz family should have different phylink_validate method from the
Peridot, since on Topaz the port supporting 2500BaseX mode is port 5,
not 9 and 10.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>