net: ethernet: mtk_eth_soc: fix tx vlan tag for llc packets

[ Upstream commit d4736737110ffa83d29f1c5d17b26113864205f6 ]

When sending llc packets with vlan tx offload, the hardware fails to
actually add the tag. Deal with this by fixing it up in software.

Fixes: 656e7052 ("net-next: mediatek: add support for MT7623 ethernet")
Reported-by: Thibaut VARENE <hacks@slashdirt.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250831182007.51619-1-nbd@nbd.name


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>