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: 6335e9f2 ("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>