rt2x00: rt61pci: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ralink/rt2x00/rt61pci.c

@@ -2226,7 +2226,7 @@ static void rt61pci_txdone(struct rt2x00_dev *rt2x00dev)
 			break;
 		case 6: /* Failure, excessive retries */
 			__set_bit(TXDONE_EXCESSIVE_RETRY, &txdesc.flags);
-			/* Don't break, this is a failed frame! */
+			/* Fall through - this is a failed frame! */
 		default: /* Failure */
 			__set_bit(TXDONE_FAILURE, &txdesc.flags);
 		}