Bluetooth: hci_conn: avoid checking uninitialized CIG/CIS ids

The CIS/CIG ids of ISO connections are defined only when the connection
is unicast.

Fix the lookup functions to check for unicast first. Ensure CIG/CIS
IDs have valid value also in state BT_OPEN.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>