Bluetooth: Controller: Revert back early abort of previous prepare

Revert back to implementation that did early abort of
previous prepare when a short prepare is enqueued.

Adds back implementation deleted in
commit 7f388bb7 ("Bluetooth: Controller: Fix short
prepare when many enqueued in pipeline").

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