Bluetooth: Controller: Fix Close Isochronous Event implementation

Fix the Close Isochronous Event (CIE) implementation so that
it follows the Bluetooth Core Specification definition:
Link Layer implementations will normally end a CIS event
early when all scheduled payloads in both directions have
been transmitted and acknowledged.

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