drivers: nrf_rtc_timer: Adjust set_absolute_alarm to prevent CC misses

This is a follow-up to commit 205e6849.

The recently added nrf_rtc_timer test case (test_next_cycle_timeouts)
revealed a problem in the current implementation of this function.
Adjust it to avoid missing COMPARE events in specific circumstances.

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