Bluetooth: controller: Prevent multiple ticker_stop in cis_disabled_cb

When calling ull_conn_iso_cis_stop with no pending LLL events,
cis_disabled_cb may be called recursively if more than one CIS is
associated with a disconnecting ACL connection.

To prevent ticker_stop being called more than once, it shall be
registered at entry of cis_disabled_cb whether this is the last CIS.
Only then shall ticker_stop be called.

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