leaudio: Fix for "Unable to get free Uni-Directional Sink CIS ID"

This could happen in the event, when CIS is not properly cleared after
device disconnection and to trigger it, CIS must be in assigned state.

Bug: 351314630
Flag: Exempt, regression tested with unit tests, new test added
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test

Change-Id: Ib13b0c511bf8f159020df050211ae9219684dc18