Bluetooth: controller: Fix XTAL_ADVANCED threshold detection

Fix incorrect calculation of unreserved timespace which
should take into account the ticks_slot of current event,
and compensation for any reduced prepare in the current
event as well as in the next event.

This regression relates to the commit ad7c9d3d76ae
("Bluetooth: controller: Improved continuous scanning")

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