Bluetooth: Controller: Fix incorrect CIS payload count at CIS Estab

Fix incorrect payload count at CIS Establish due to existing
CIG event overlapping the ACL event at the instant when the
CIS gets the active flag set.

The overlapping CIG event picked up the new CIS that had its
active flag set in the current CIG event instead of at the
actual CIS offset which is in the next CIG event.

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