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 <pab@pabigot.com>
d2f507855b