net: ethernet: mediatek: Enable GDM GDMA_DROP_ALL mode

Enable GDM GDMA_DROP_ALL mode to drop all packet during the
stop operation. This is recommended by the mt762x HW design
to drop all packet from GMAC before stopping PDMA.

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