net: dsa: mv88e6xxx: SERDES support 2500BaseT via external PHY

By using an external PHY, ports 9 and 10 can support 2500BaseT.
So set this link mode in the mask when validating.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>