net: dsa: mv88e6xxx: don't error out cmode set on missing lane

When the given cmode has no serdes, mv88e6xxx_serdes_get_lane() returns
-NODEV. Earlier in the same function the code skips serdes handing in
this case. Do the same after cmode set.
Signed-off-by: NBaruch Siach <baruch.siach@siklu.com>
Link: https://lore.kernel.org/r/cd95cf3422ae8daf297a01fa9ec3931b203cdf45.1646050203.git.baruch@tkos.co.ilSigned-off-by: NJakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6xxx/port.c

@@ -610,6 +610,8 @@ static int mv88e6xxx_port_set_cmode(struct mv88e6xxx_chip *chip, int port,
 		chip->ports[port].cmode = cmode;
 
 		lane = mv88e6xxx_serdes_get_lane(chip, port);
+		if (lane == -ENODEV)
+			return 0;
 		if (lane < 0)
 			return lane;