fm10k: add missing fall through comment

Newer versions of GCC starting with 7 now additionally warn when a case
statement may fall through without an explicit comment mentioning it.
Add such a comment to silence the warning, as this is expected.

Unfortunately the comment must come directly before the next case
statement, so we put it outside the #ifdef. Otherwise, the compiler
cannot properly detect it and thus the warning is displayed regardless.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/fm10k/fm10k_main.c

@@ -876,6 +876,7 @@ static void fm10k_tx_csum(struct fm10k_ring *tx_ring,
 	case IPPROTO_GRE:
 		if (skb->encapsulation)
 			break;
+		/* fall through */
 	default:
 		if (unlikely(net_ratelimit())) {
 			dev_warn(tx_ring->dev,