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>