Input: xen-kbdfront - mark expected switch fall-through

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

Notice that in this particular case, I placed the "fall through"
part at the beginning of the code comment, which is what GCC is
expecting to find.

Addresses-Coverity-ID: 114757 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/misc/xen-kbdfront.c

@@ -524,7 +524,7 @@ static void xenkbd_backend_changed(struct xenbus_device *dev,
 	case XenbusStateClosed:
 		if (dev->state == XenbusStateClosed)
 			break;
-		/* Missed the backend's CLOSING state -- fallthrough */
+		/* fall through - Missed the backend's CLOSING state */
 	case XenbusStateClosing:
 		xenbus_frontend_closed(dev);
 		break;