watchdog: machzwd : fix warning Using plain integer as NULL pointer

Changes passing function argument 0 to NULL to avoid below sparse
warning

 CHECK   drivers/watchdog//machzwd.c
drivers/watchdog//machzwd.c:321:25: warning: Using plain integer as NULL
pointer
Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/machzwd.c

@@ -318,7 +318,7 @@ static long zf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case WDIOC_GETBOOTSTATUS:
 		return put_user(0, p);
 	case WDIOC_KEEPALIVE:
-		zf_ping(0);
+		zf_ping(NULL);
 		break;
 	default:
 		return -ENOTTY;