net: dsa: mv88e6xxx: Unlock on error in mv88e6xxx_port_bridge_join()

Call mv88e6xxx_reg_unlock(chip) before returning on this error path.

Fixes: 7af4a361 ("net: dsa: mv88e6xxx: Improve isolation of standalone ports")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>