igb_main: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/net/ethernet/intel/igb/igb_main.c: In function ‘__igb_notify_dca’:
drivers/net/ethernet/intel/igb/igb_main.c:6694:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

   if (dca_add_requester(dev) == 0) {
      ^

drivers/net/ethernet/intel/igb/igb_main.c:6701:2: note: here
case DCA_PROVIDER_REMOVE:
^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable -Wimplicit-fallthrough.

---
 drivers/net/ethernet/intel/igb/igb_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 69b230c53fed..483f0977f70e 100644
@@ -6697,7 +6697,7 @@ static int __igb_notify_dca(struct device *dev, void *data)
 			igb_setup_dca(adapter);
 			break;
 		}
-		/* Fall Through since DCA is disabled. */
+		/* Fall Through - since DCA is disabled. */
 	case DCA_PROVIDER_REMOVE:
 		if (adapter->flags & IGB_FLAG_DCA_ENABLED) {
 			/* without this a class_device is left

2.20.1