net: ethernet: mtk_eth_wed: add missing of_node_put()

The device_node pointer returned by of_parse_phandle() with refcount
incremented, when finish using it, the refcount need be decreased.

Fixes: 804775df ("net: ethernet: mtk_eth_soc: add support for Wireless Ethernet Dispatch (WED)")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>