net: dsa: mv88e6xxx: Differentiate between 6390 and 6390X cmodes

The X family variants support additional ports modes, for 10G
operation, which the non-X variants don't have. Add a port_set_cmode()
for non-X variants to enforce this.

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