clockevents/drivers/timer-stm32: Fix build warning spotted by kbuild test robot

This patch fixes below warning spotted by kbuild test robot when building
with ARCH=powerpc:

   drivers/clocksource/timer-stm32.c: In function 'stm32_clockevent_init':
>> drivers/clocksource/timer-stm32.c:140:9: warning: large integer implicitly
	truncated to unsigned type [-Woverflow]

     writel_relaxed(~0UL, data->base + TIM_ARR);

The fix consists in using 0U instead of 0UL.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>