watchdog: wdat_wdt: Fix warning for using 0 as NULL

Fixes the following sparse warnings:

drivers/watchdog/wdat_wdt.c:210:66: warning: Using plain integer as NULL pointer
drivers/watchdog/wdat_wdt.c:235:66: warning: Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>