Bluetooth: controller: nRF5: Check preempt event on timeout

Check whether the preempt event matches with the head of the
pipeline before aborting the currently active event.

This is required to avoid preemption of events that became
active due to done event and there has been a race in
stopping the preempt ticker.

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