net: stmmac: dwmac-sun8i: add missing of_node_put() in sun8i_dwmac_register_mdio_mux()

The node pointer returned by of_get_child_by_name() with refcount incremented,
so add of_node_put() after using it.

Fixes: 634db83b

 ("net: stmmac: dwmac-sun8i: Handle integrated/external MDIOs")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220428095716.540452-1-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>