net: dsa: mv88e6xxx: Refactor SERDES lane code

The 6390 family has 8 SERDES lanes. What ports use these lanes depends
on how ports 9 and 10 are configured. If 9 and 10 does not make use of
a line, one of the lower ports can use it.

Add a function to return the lane a port is using, if any, and simplify
the code to power up/down the lane.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>