net: dsa: mv88e6xxx: setup message port is not supported in the 6250 familiy

The MV88E6250 family doesn't support the MV88E6XXX_PORT_CTL1_MESSAGE_PORT
bit.

Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>