Bluetooth: ISO: Fix crash when channel has already been disconnected

If chan->conn is already NULL do not call bt_conn_unref as that will
likely cause a crash, also this make sure that if channel has been
disconnected using bt_iso_chan_disconnect it removes the channel from
connection list before setting the chan->conn to NULL.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>