net: dsa: mv88e6xxx: handle SERDES error appropriately

mv88e6xxx_serdes_power returns an error, so no need to print an error
message inside of it. Rather print it in its caller when the error is
ignored, which is in the mv88e6xxx_port_disable void function.

Catch and return its error in the counterpart mv88e6xxx_port_enable.

Fixes: 04aca993 ("dsa: mv88e6xxx: Enable/Disable SERDES on port enable/disable")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>