Bluetooth: clean up connection in hci_cs_disconnect

In bluetooth core specification 4.2,
Vol 2, Part E, 7.8.9 LE Set Advertise Enable Command, it says

    The Controller shall continue advertising until ...
    or until a connection is created or ...
    In these cases, advertising is then disabled.

Hence, advertising would be disabled before a connection is
established. In current kernel implementation, advertising would
be re-enabled when all connections are terminated.

The correct disconnection flow looks like

  < HCI Command: Disconnect

  > HCI Event: Command Status
      Status: Success

  > HCI Event: Disconnect Complete
      Status: Success

Specifically, the last Disconnect Complete Event would trigger a
callback function hci_event.c:hci_disconn_complete_evt() to
cleanup the connection and re-enable advertising when proper.

However, sometimes, there might occur an exception in the controller
when disconnection is being executed. The disconnection flow might
then look like

  < HCI Command: Disconnect

  > HCI Event: Command Status
      Status: Unknown Connection Identifier

  Note that "> HCI Event: Disconnect Complete" is missing when such an
exception occurs. This would result in advertising staying disabled
forever since the connection in question is not cleaned up correctly.

To fix the controller exception issue, we need to do some connection
cleanup when the disconnect command status indicates an error.

Signed-off-by: Joseph Hwang <josephsih@chromium.org>
Signed-off-by: Manish Mandlik <mmandlik@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>