watchdog: w83977f_wdt: 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: NGustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

drivers/watchdog/w83977f_wdt.c

@@ -427,7 +427,7 @@ static long wdt_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			return -EINVAL;
 
 		wdt_keepalive();
-		/* Fall */
+		/* Fall through */
 
 	case WDIOC_GETTIMEOUT:
 		return put_user(timeout, uarg.i);