tsnep: Add missing of_node_put() in tsnep_mdio_init()

The node pointer is returned by of_get_child_by_name() with
refcount incremented in tsnep_mdio_init(). Calling of_node_put()
to aovid the refcount leak in tsnep_mdio_init().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20211124084048.175456-1-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>