Bluetooth: Controller: Fix BT_CTLR_EARLY_ABORT_PREVIOUS_PREPARE

Fix BT_CTLR_EARLY_ABORT_PREVIOUS_PREPARE to setup new
shorter preempt timeout irrespective of whether there
is a previous prepare enqueued in the prepare pipeline.

Relates to commit d573951f ("Bluetooth: Controller:
Revert back early abort of previous prepare").

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