watchdog: sp5100_tco.c: quiet sparse noise about using plain integer was NULL pointer

Pointers should not be compared to plain integers.
Quiets the sparse warning:
warning: Using plain integer as NULL pointer

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>