drivers: timer: nrf_rtc_timer: fix lost ticks from unannounced elapsed

The original code assumed that limiting the tick count to the maximum
cycle value representable without wrapping would guarantee that adding
the resulting cycle offset to last_count would not lap the counter.
This is not true when elapsed time, which is also added to the cycle
offset, exceeds one tick.  Cap the maximum offset at the number of
cycles corresponding to the maximum number of ticks without wrapping.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>