drivers: timer: nrf_rtc_timer: fix handling for 24-bit counter
drivers: timer: nrf_rtc_timer: fix handling for 24-bit counter

Two subtractions failed to account for the possibility that a calculated
time exceeded the counter resolution, allowing a comparison to
improperly indicate that a minimum delay was satisfied.

Use the subtraction helper to avoid the problem.

(The subtraction in z_clock_set_timeout was the cause of issue #11694;
the one in rtc1_nrf5_isr was replaced based on inspection rather than
testing.)

Closes #11694

Signed-off-by: Peter A. Bigot <650538893330be855f76604594b4820eb226769e@pabigot.com>

zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr