drivers: counter: fix nRF prescaler overflow

use u32_t in init_rtc to be able to use the full 12 bit prescaler values
fixes zephyrproject-rtos#22014

Signed-off-by: Christian Hirsch <christian.hirsch@tuwien.ac.at>

drivers/counter/counter_nrfx_rtc.c

@@ -516,7 +516,7 @@ static u32_t get_pending_int(struct device *dev)
 	return 0;
 }
 
-static int init_rtc(struct device *dev, u8_t prescaler)
+static int init_rtc(struct device *dev, u32_t prescaler)
 {
 	struct device *clock;
 	const struct counter_nrfx_config *nrfx_config = get_nrfx_config(dev);