net: ethernet: mediatek: add the missing of_node_put() after node is used done

This patch adds the missing of_node_put() after finishing the usage
of of_parse_phandle() or of_node_get() used by fixed_phy.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>