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: NWei Yongjun <weiyongjun1@huawei.com>
Acked-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>