net: lan78xx: Reduce s/w csum check on VLANs

With HW_VLAN_CTAG_RX enabled we don't observe the checksum
issue, so amend the workaround to only drop back to s/w
checksums if VLAN offload is disabled.

See #2458.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>

drivers/net/usb/lan78xx.c

@@ -2997,12 +2997,13 @@ static void lan78xx_rx_csum_offload(struct lan78xx_net *dev,
 				    struct sk_buff *skb,
 				    u32 rx_cmd_a, u32 rx_cmd_b)
 {
-	/* Checksum offload appears to be flawed if used with VLANs.
-	 * Elect for sw checksum check instead.
+	/* HW Checksum offload appears to be flawed if used when not stripping
+	 * VLAN headers.
 	 */
 	if (!(dev->net->features & NETIF_F_RXCSUM) ||
 	    unlikely(rx_cmd_a & RX_CMD_A_ICSM_) ||
-	    (rx_cmd_a & RX_CMD_A_FVTG_)) {
+	    ((rx_cmd_a & RX_CMD_A_FVTG_) &&
+	     !(dev->net->features & NETIF_F_HW_VLAN_CTAG_RX))) {
 		skb->ip_summed = CHECKSUM_NONE;
 	} else {
 		skb->csum = ntohs((u16)(rx_cmd_b >> RX_CMD_B_CSUM_SHIFT_));