net: dsa: mv88e6xxx: 2500Base-X uses the 1000Base-X SERDES

The 6390 has three different SERDES interface types. 2500Base-X is
implemented by the SGMII/1000Base-X SERDES. So power on/off the
correct SERDES.

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