net: dsa: mv88e6xxx: fix error code in mv88e6390_serdes_power()

We're accidentally returning the wrong variable.  "cmode" is
uninitialized at this point so it causes a static checker warning.

Fixes: 6335e9f2446b ("net: dsa: mv88e6xxx: mv88e6390X SERDES support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/serdes.c

@@ -192,7 +192,7 @@ int mv88e6390_serdes_power(struct mv88e6xxx_chip *chip, int port, bool on)
 
 	err = mv88e6xxx_port_get_cmode(chip, port, &cmode);
 	if (err)
-		return cmode;
+		return err;
 
 	switch (port) {
 	case 2: