stmmac: dwmac-loongson: fix missing of_node_put() while module exiting

The node returned by of_get_child_by_name() with refcount decremented,
of_node_put() needs be called when finish using it. So add it in the
error path in loongson_dwmac_probe() and in loongson_dwmac_remove().

Fixes: 2ae34111 ("stmmac: dwmac-loongson: fix invalid mdio_node")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>