net: ethernet: mtk_eth_soc: enable hardware DSA untagging

- pass the tag to DSA via metadata dst
- disabled on 7986 for now, since it's not working yet
- disabled if a MAC is enabled that does not use DSA

This improves performance by bypassing the DSA tag driver and avoiding extra
skb data mangling

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>