Bluetooth: controller: Keep CIS/CIG instances until ll_cig_remove

This commit enables having multiple CIS/CIG setups/connects and
teardowns in sequence.

- For central, cig->lll.num_cis is initialized to cis_count and
  untouched until ll_cig_remove. The value shall indicate number of
  allocated instances, to ensure correct CIG traversal.
- Keep CIG/CIS instances for central until ll_cig_remove
- Initialize CIS instance state vars in ll_cis_create, similar to the
  peripheral flow.
- In ll_cig_remove, always release streams and CIG instance. Do not
  remove paths, as this is done from host at bt_iso_chan_disconnected.
- Remove unsued cis_count member in ll_conn_iso_group.
- Use correct function ull_iso_lll_ack_enqueue at LLL flush to have
  data path respected.

Signed-off-by: Morten Priess <mtpr@oticon.com>