net: dsa: mv88e6xxx: Fix 6095/6097/6185 ports in non-SERDES CMODE

The .serdes_get_lane op used the magic value 0xff to indicate a valid
SERDES lane and 0 signaled that a non-SERDES mode was set on the port.

Unfortunately, "0" is also a valid lane ID, so even when these ports
where configured to e.g. RGMII the driver would set them up as SERDES
ports.

- Replace 0xff with 0 to indicate a valid lane ID. The number is on
  the one hand just as arbitrary, but it is at least the first valid one
  and therefore less of a surprise.

- Follow the other .serdes_get_lane implementations and return -ENODEV
  in the case where no SERDES is assigned to the port.

Fixes: f5be107c ("net: dsa: mv88e6xxx: Support serdes ports on MV88E6097/6095/6185")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>