qcacmn: htc: Replace fallthrough comment with attribute

-Wimplicit-fallthrough is being enabled by default. Some compilers
such as clang require the attribute instead of just a fallthrough comment.

Change-Id: I7ed1baaad93aea2cd2551dd3b5f9a93953154188
CRs-Fixed: 3218240

htc/htc_send.c

@@ -765,7 +765,7 @@ static QDF_STATUS htc_issue_packets(HTC_TARGET *target,
 					    (pEndpoint) < 1)
 						break;
 				}
-				/* fallthrough */
+				fallthrough;
 			case QDF_BUS_TYPE_USB:
 				htc_issue_packets_bundle(target,
 							pEndpoint,
@@ -1681,7 +1681,7 @@ static enum HTC_SEND_QUEUE_RESULT htc_try_send(HTC_TARGET *target,
 					pEndpoint->total_num_requeues++;
 					pEndpoint->num_requeues_warn = 0;
 				}
-				/* fallthrough */
+				fallthrough;
 			default:
 				QDF_TRACE(QDF_MODULE_ID_HIF, QDF_TRACE_LEVEL_INFO,
 					  "htc_issue_packets, failed status:%d"