clocksource/drivers/timer-microchip-pit64b: Fix sparse warning

Fix sparse warning:
"warning: Using plain integer as NULL pointer"

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1578304688-14882-1-git-send-email-claudiu.beznea@microchip.com