Bluetooth: Controller: Fix preempt timeout not skip first in pipeline

Fix preempt timeout scheduling to not skip first prepare in
pipeline when there are no previous preempt timeout
scheduled already.

Fix related to commit 27b8beaa ("Bluetooth: Controller:
Fix to handle relative short preempt timeout").

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