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

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: Weijie Gao <weijie.gao@mediatek.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>