Bluetooth: ISO: Handle central disc of pending CIS

If the central disconnects a CIS while it is being
established, then we receive both a CIS established event
with BT_HCI_ERR_OP_CANCELLED_BY_HOST and a disconnect complete
event.

In this case we should not call bt_iso_disconnected in the
CIS established event handler, as that will also be
called from the disconnect complete event handler.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>