net: ethernet: mtk_eth_soc: avoid port_mg assignment on MT7622 and newer

On newer chips, this field is unused and contains some bits related to queue
assignment. Initialize it to 0 in those cases.
Fix offload_version on MT7621 and MT7623, which still need the previous value.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20221116080734.44013-4-nbd@nbd.name
Signed-off-by: Jakub Kicinski <kuba@kernel.org>