net: ethernet: mediatek: Refine the timing of GDM/PSE setup

Refine the timing of GDM/PSE setup, move it from mtk_hw_init
to mtk_open. This is recommended by the mt762x HW design to
do GDM/PSE setup only after PDMA has been started.

We exclude mt7628 in mtk_gdm_config function since it is a old IP
and there is no GDM/PSE block on it.

Signed-off-by: MarkLee <Mark-MC.Lee@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>