Bluetooth: ISO: Don't call LE Create Conn Cancel on ISO disconnect

When calling `bt_iso_chan_disconnect()` while the channel is
connecting, the host should simply use HCI Disconnect.

Note: For the peripheral it is not allowed to call HCI Disconnect
on a CIS after it has accepted the CIS request, but before it has
received the CIS established event. The implementation in this
commit does not fix this case. In that case the controller will
return an error code.

Also, this commit does not handle the events following the
HCI Disconnect correctly.
In this particular case, two events are raised by the controller.
See Core_v5.4, Vol 6, Part D, Figure 6.51.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>