net: ethernet: mtk_eth_soc: fix RX VLAN offload

[ Upstream commit 3f57d8c40fea9b20543cab4da12f4680d2ef182c ]

The VLAN ID in the rx descriptor is only valid if the RX_DMA_VTAG bit is
set. Fixes frames wrongly marked with VLAN tags.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
[Ilya: fix commit message]
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>