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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NMaxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/timer-stm32.c

@@ -137,9 +137,9 @@ static void __init stm32_clockevent_init(struct device_node *np)
 	}
 
 	/* Detect whether the timer is 16 or 32 bits */
-	writel_relaxed(~0UL, data->base + TIM_ARR);
+	writel_relaxed(~0U, data->base + TIM_ARR);
 	max_delta = readl_relaxed(data->base + TIM_ARR);
-	if (max_delta == ~0UL) {
+	if (max_delta == ~0U) {
 		prescaler = 1;
 		bits = 32;
 	} else {