net: dsa: mv88e6xxx: implement mv88e6352_serdes_get_lane

Even though 88E6352 has no dedicated lane for SERDES interfaces, it
uses a similar code as the other .serdes_get_lane implementations to
check the port's CMODE and ensure that SERDES operations are doable.

For consistency, implement mv88e6352_serdes_get_lane for the 88E6352
and similar switches which simply returns an unused 0xff lane address.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>