drivers: nrf_rtc_timer: Always announce actual number of elapsed ticks

In non-tickless mode, the timeout handler always announced maximum 1
tick to kernel, but in fact it cannot be guaranteed that the handler
execution is not delayed and that the number of elapsed ticks does not
exceed 1. Use the actual number instead.
Switch also to using a 32-bit value for `dticks` to get a bit simpler
generated code (ticks delta is not supposed to be that huge).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>