Bluetooth: Controller: Fix short prepare timeout start (2nd attempt)

Fix to setup correct short prepare timeout when preempt
callback does not find the prepare in the head of the
pipeline. In this case, find the short prepare and setup
a fresh preempt timeout for the determined short prepare
present in the pipeline.

Relates to commit 7d1bc178 ("Bluetooth: Controller: Fix
short prepare preempt timeout start").

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