Bluetooth: Controller: Fix Peripheral CIS sorted by CIG

Fix Peripheral CIS sorted by CIG implemenation to use CIS
offset stored in LLL context which is the correct offset
from the CIG anchor point. CIS offset in the ULL context
is the offset from the ACL anchor point at the time of
the CIS establishment.

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