net: dsa: mv88e6xxx: merge mv88e6352_serdes_power_set

The mv88e6352_serdes_power_set helper is only used at one place, in
mv88e6352_serdes_power. Keep it simple and merge the two functions
together.

Use mv88e6xxx_serdes_get_lane instead of mv88e6352_port_has_serdes
to avoid moving code. No functional changes.

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