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>

drivers/net/dsa/mv88e6xxx/serdes.c

@@ -443,15 +443,15 @@ int mv88e6185_serdes_power(struct mv88e6xxx_chip *chip, int port, int lane,
 int mv88e6185_serdes_get_lane(struct mv88e6xxx_chip *chip, int port)
 {
 	/* There are no configurable serdes lanes on this switch chip but we
-	 * need to return non-zero so that callers of
+	 * need to return a non-negative lane number so that callers of
 	 * mv88e6xxx_serdes_get_lane() know this is a serdes port.
 	 */
 	switch (chip->ports[port].cmode) {
 	case MV88E6185_PORT_STS_CMODE_SERDES:
 	case MV88E6185_PORT_STS_CMODE_1000BASE_X:
-		return 0xff;
-	default:
 		return 0;
+	default:
+		return -ENODEV;
 	}
 }