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>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -1319,7 +1319,7 @@ static int mtk_poll_rx(struct napi_struct *napi, int budget,
 		skb->protocol = eth_type_trans(skb, netdev);
 
 		if (netdev->features & NETIF_F_HW_VLAN_CTAG_RX &&
-		    RX_DMA_VID(trxd.rxd3))
+		    (trxd.rxd2 & RX_DMA_VTAG))
 			__vlan_hwaccel_put_tag(skb, htons(ETH_P_8021Q),
 					       RX_DMA_VID(trxd.rxd3));
 		skb_record_rx_queue(skb, 0);

drivers/net/ethernet/mediatek/mtk_eth_soc.h

@@ -301,6 +301,7 @@
 #define RX_DMA_LSO		BIT(30)
 #define RX_DMA_PLEN0(_x)	(((_x) & 0x3fff) << 16)
 #define RX_DMA_GET_PLEN0(_x)	(((_x) >> 16) & 0x3fff)
+#define RX_DMA_VTAG		BIT(15)
 
 /* QDMA descriptor rxd3 */
 #define RX_DMA_VID(_x)		((_x) & 0xfff)