net: dsa: mv88e6xxx: missing unlock on error path

We added a new error path, but we need to drop the lock before we return.

Fixes: 2d2e1dd2 ("net: dsa: mv88e6xxx: Cache the port cmode")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>