Fix l2cu_initialize_fixed_ccb() to return only initialized entries

Update the l2cu_initialize_fixed_ccb() implementation so
it returns only entries that are properly initialized.
In some complex scenarios, the p_lcb->p_fixed_ccbs[] array
might contain an entry that has been released by l2cu_release_ccb(),
and such entry cannot be reused as-is.

Bug: 26927945
Change-Id: I42d42926cc6da6b8be662781545753c36b75968e