usb: musb: add code comment for clarification

Add code comment to make it clear that the fall-through is intentional.
Read the link for more details: https://lkml.org/lkml/2017/2/9/292

Addresses-Coverity-ID: 1397608
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_core.c

@@ -1869,6 +1869,7 @@ static void musb_pm_runtime_check_session(struct musb *musb)
 
 			return;
 		}
+		/* fall through */
 	case MUSB_QUIRK_A_DISCONNECT_19:
 		if (musb->quirk_retries--) {
 			musb_dbg(musb,