net: dsa: mv88e6xxx: Refactor mv88e6352 SERDES code into an op

The mv88e6390 family has a different SERDES implementation. Refactor
the mv88e6352 code into an ops function, so we can later add the
mv88e6390 code.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>