Now the first step of all .serdes_power implementations is getting
the lane mapping. Since we have an operation for that, call it in
the wrapper and pass the lane down to the .serdes_power operation.

This also allows to avoid querying the SERDES lane twice in
mv88e6xxx_port_set_cmode.

At the same time provide mv88e6xxx_serdes_power_{up,down} helpers
and prefer up/down instead of on/off as in the documentation.
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>