eth: mtk-eth: fix incorrect read of phy-handle

In mt7629-rfb.dts, the phy-handle is a reference to the node phy0, not the
node itself:

	phy-handle = <&phy0>;

	phy0: ethernet-phy@0 {
		reg = <0>;
	}

However the driver used ofnode_find_subnode("phy-handle") to read the node.
It will always fail.

This patch replaces ofnode_find_subnode with dev_read_phandle_with_args to
make sure the node can be read correctly.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: NWeijie Gao <weijie.gao@mediatek.com>
Acked-by: NJoe Hershberger <joe.hershberger@ni.com>

drivers/net/mtk_eth.c

@@ -1130,13 +1130,14 @@ static int mtk_eth_ofdata_to_platdata(struct udevice *dev)
 					     &priv->rst_gpio, GPIOD_IS_OUT);
 		}
 	} else {
-		subnode = ofnode_find_subnode(dev_ofnode(dev), "phy-handle");
-		if (!ofnode_valid(subnode)) {
+		ret = dev_read_phandle_with_args(dev, "phy-handle", NULL, 0,
+						 0, &args);
+		if (ret) {
 			printf("error: phy-handle is not specified\n");
 			return ret;
 		}
 
-		priv->phy_addr = ofnode_read_s32_default(subnode, "reg", -1);
+		priv->phy_addr = ofnode_read_s32_default(args.node, "reg", -1);
 		if (priv->phy_addr < 0) {
 			printf("error: phy address is not specified\n");
 			return ret;