net: ethernet: mediatek: fix the missing of_node_put() after node is used done inside mtk_mdio_init

This patch adds the missing of_node_put() after finishing the usage
of of_get_child_by_name.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>