net: dsa: mv88e6xxx: Power on/off SERDES on cmode change

The 6390 family has a number of SERDES interfaces per port. When the
cmode changes, eg 1000Base-X to XAUI, the SERDES interface in use will
also change. Power down the old SERDES interface and power up the new
SERDES interface.

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