Bluetooth: Controller: Fix to handle relative short preempt timeout

When a new radio event is scheduled with a relatively short
preempt timeout while there is already a started preempt
ticker, then stop the ticker, abort the previous event that
requested the preemption, and request to start ticker with
the new relatively short preempt timeout.

Fixes #30245.

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