Bluetooth: ISO: Fix CIG auto-allocation to select configurable CIG

Make CIG auto-allocation to select the first CIG_ID that is still
configurable. Also use correct CIG_ID range (see Core v5.3 Vol 4 Part E
Sec 7.8.97 p.2553).

Previously, it would always select CIG_ID 0 regardless of anything,
because cis_list with data.cis == 0xff (BT_ISO_QOS_CIS_UNSET) would not
count any CIS. Since we are not adding CIS here, use find_cis instead.

Fixes: 26afbd82 ("Bluetooth: Add initial implementation of CIS connections")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>