net: dsa: mv88e6xxx: Set correct interface mode for CPU/DSA ports

By default, the switch driver is expected to configure CPU and DSA
ports to their maximum speed. For the 6341 and 6390 families, the
ports interface mode has to be configured as well. The 6390X range
support 10G ports using XAUI, while the 6341 and 6390 supports
2500BaseX, as their maximum speed.

Fixes: 787799a9

 ("net: dsa: mv88e6xxx: Default ports 9/10 6390X CMODE to 1000BaseX")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>