net: dsa: mv88e6xxx: simplify .serdes_get_lane

Because the mapping between a SERDES interface and its lane is static,
we don't need to stick with negative error codes actually and we can
simply return 0 if there is no lane, just like the IRQ mapping.

This way we can keep a simple and intuitive API using unsigned lane
numbers while simplifying the implementations with single return
statements. Last but not least, fix the reverse chrismas tree in
mv88e6390x_serdes_get_lane.

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