Bluetooth: Controller: Use reschedule margin as minimum ticks_slot

Use reschedule margin as minimum ticks_slot when ticker with
reschedule expires. This will ensure not too many unreserved
tickers expire in close proximity that can lead of pile up
of CPU processing time and eventually causing timeout
callbacks to be delayed. This mitigate possible LL asserts
due to overhead in start of radio events.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>