Bluetooth: Controller: Fix Central ISO establish assertion failure

Fix Central ISO assetion failure on being established due to
regression in commit a53d8945 ("Bluetooth: controller:
fixing CIS establish flow for refactored LLCP") that
required changes due to Create CIS LLCP being refactored.

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