net: ethernet: mtk_eth_soc: fix RX VLAN offload

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>