Bluetooth: Controller: Fix assertion failed [evdone]

When there are radio events with time reservations lower
than the preemption timeout of 1.5 ms, the pipeline has to
account for the maximum radio events that can be enqueued
during the preempt timeout duration. All these enqueued
events could be aborted in case of late scheduling needing
as many done event buffers.

During continuous scanning, there can be 1 active radio
event, 1 scan resume and 1 new scan prepare. If there are
peripheral prepares in addition, and due to late scheduling
all these will abort needing 4 done buffers.

If Extended Scanning is supported, then an additional
auxiliary scan event's prepare could be enqueued in the
pipeline during the preemption duration.

Fixes #36381.

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