net: dsa: mv88e6xxx: Enable CMODE config support for 6390X

Commit f39908d3 ('net: dsa: mv88e6xxx: Set the CMODE for mv88e6390
ports 9 & 10') added support for setting the CMODE for the 6390X family,
but only enabled it for 9290 and 6390 - and left out 6390X.

Fix support for setting the CMODE on 6390X also by assigning
mv88e6390x_port_set_cmode() to the .port_set_cmode function pointer in
mv88e6390x_ops too.

Fixes: f39908d3

 ("net: dsa: mv88e6xxx: Set the CMODE for mv88e6390 ports 9 & 10")
Signed-off-by: Martin Hundebøll <mnhu@prevas.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>