net: dsa: mv88e6xxx: create serdes_get_lane chip operation

Create a serdes_get_lane() method in the mv88e6xxx operations structure.
Use it instead of calling the different implementations.
Also change the methods so that their return value is used only for
error. The lane number is put into a place referred to by a pointer
given as argument. If the port does not have a lane, return -ENODEV.
Lanes are phy addresses, so use u8 as their type.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>