net: dsa: mv88e6xxx: Fix refcount leak in mv88e6xxx_mdios_register

of_get_child_by_name() returns a node pointer with refcount
incremented, we should use of_node_put() on it when done.

mv88e6xxx_mdio_register() pass the device node to of_mdiobus_register().
We don't need the device node after it.

Add missing of_node_put() to avoid refcount leak.

Fixes: a3c53be5

 ("net: dsa: mv88e6xxx: Support multiple MDIO busses")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>