watchdog: pcwd_usb: 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 replaced "Fall" with a proper
"fall through" comment, which is what GCC is expecting to find.
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/pcwd_usb.c

@@ -456,8 +456,8 @@ static long usb_pcwd_ioctl(struct file *file, unsigned int cmd,
 			return -EINVAL;
 
 		usb_pcwd_keepalive(usb_pcwd_device);
-		/* Fall */
 	}
+		/* fall through */
 
 	case WDIOC_GETTIMEOUT:
 		return put_user(heartbeat, p);