net: ethernet: mediatek: modify GDM to send packets to the PDMA for RX

Because we change to use the PDMA as the Ethernet RX DMA engine,
the patch modifies to set GDM to send packets to PDMA for RX.

Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Nelson Chang <nelson.chang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>