drivers: nrf_rtc_timer: Simplify sys_clock_set_timeout calculations

Remove unnecessary decreasing of the number of ticks by 1 (it was then
increased by 1 when it was converted to the number of cycles) and add
a comment that clarifies the way that ticks < 1 are handled.

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