net: ethernet: mtk_eth_soc: fix parsing packets in GDM

When using DSA, set the special tag in GDM ingress control to allow the MAC
to parse packets properly earlier. This affects rx DMA source port reporting.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>