igbvf: netdev: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 114801 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igbvf/netdev.c

@@ -2102,6 +2102,7 @@ csum_failed:
 			type_tucmd = E1000_ADVTXD_TUCMD_L4T_SCTP;
 			break;
 		}
+		/* fall through */
 	default:
 		skb_checksum_help(skb);
 		goto csum_failed;