With this it is possible to mark certain chip ports as invalid. This is
required for example for the MV88E6220 (which is in general a MV88E6250
with 7 ports) but the ports 2-4 are not routed to pins.

If a user configures an invalid port, an error is returned.
Signed-off-by: NHubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>