Bluetooth: Classic: SCO: Get SCO conn info

Save the air_mode to SCO connect object.

Add a structure `struct bt_conn_sco_info` to return the SCO conn info.

Return SCO info through `struct bt_conn_sco_info` when calling the
function `bt_conn_get_info()`.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>