cnss2: Replace fallthrough comment with attribute

Some compilers such as clang requires the attribute instead
of just a fallthrough comment when -Wimplicit-fallthrough
is enabled. So replace fallthrough comment with attribute.

Change-Id: I1935bfb325b550fbc6539c58a99b3696fb6cc753
CRs-Fixed: 3295197

cnss2/main.c

@@ -2291,14 +2291,14 @@ static void cnss_driver_event_work(struct work_struct *work)
 		case CNSS_DRIVER_EVENT_IDLE_RESTART:
 			set_bit(CNSS_DRIVER_IDLE_RESTART,
 				&plat_priv->driver_state);
-			/* fall through */
+			fallthrough;
 		case CNSS_DRIVER_EVENT_POWER_UP:
 			ret = cnss_power_up_hdlr(plat_priv);
 			break;
 		case CNSS_DRIVER_EVENT_IDLE_SHUTDOWN:
 			set_bit(CNSS_DRIVER_IDLE_SHUTDOWN,
 				&plat_priv->driver_state);
-			/* fall through */
+			fallthrough;
 		case CNSS_DRIVER_EVENT_POWER_DOWN:
 			ret = cnss_power_down_hdlr(plat_priv);
 			break;