Bluetooth: controller: split: Fix densely scheduled event preemption

Fix LLL implementation handling preemption of currently
active radio event with densely scheduled events in the
pipeline.

Preempt timeout was stopped without consideration to there
being more queued events in the pipeline. Also, added
chaining of preemption timeouts one after the other expiry
so as to preempt currently active events by the densely
scheduled events in the pipeline.

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