Bluetooth: Controller: Fix prepare pipeline preempt timeout stop

Fix prepare pipeline preempt timeout stop from stopping a
a prevously started timeout for the next in pipeline event.

The preempt timeout started prior for the current event that
has its prepare that is invoked is to be not explicitly
stopped. If there is a next prepare event in pipeline, then
the prior preempt timeout if started will be stopped before
starting the new preempt timeout. Refer to implementation in
preempt_ticker_start().

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