Bluetooth: hci_event: Treat UNKNOWN_CONN_ID on disconnect as success

Bluetooth: hci_event: Treat UNKNOWN_CONN_ID on disconnect as success

When the host sends an HCI_OP_DISCONNECT command, the controller may
respond with the status HCI_ERROR_UNKNOWN_CONN_ID (0x02). E.g. this can
happen on resume from suspend, if the link was terminated by the remote
device before the event mask was correctly set.

This is a btmon snippet that shows the issue:
```
> ACL Data RX: Handle 3 flags 0x02 dlen 12
      L2CAP: Disconnection Request (0x06) ident 5 len 4
        Destination CID: 65
        Source CID: 72
< ACL Data TX: Handle 3 flags 0x00 dlen 12
      L2CAP: Disconnection Response (0x07) ident 5 len 4
        Destination CID: 65
        Source CID: 72
> ACL Data RX: Handle 3 flags 0x02 dlen 12
      L2CAP: Disconnection Request (0x06) ident 6 len 4
        Destination CID: 64
        Source CID: 71
< ACL Data TX: Handle 3 flags 0x00 dlen 12
      L2CAP: Disconnection Response (0x07) ident 6 len 4
        Destination CID: 64
        Source CID: 71
< HCI Command: Set Event Mask (0x03|0x0001) plen 8
        Mask: 0x3dbff807fffbffff
          Inquiry Complete
          Inquiry Result
          Connection Complete
          Connection Request
          Disconnection Complete
          Authentication Complete
[...]
< HCI Command: Disconnect (0x01|0x0006) plen 3
        Handle: 3 Address: 78:20:A5:4A:DF:28 (Nintendo Co.,Ltd)
        Reason: Remote User Terminated Connection (0x13)
> HCI Event: Command Status (0x0f) plen 4
      Disconnect (0x01|0x0006) ncmd 1
        Status: Unknown Connection Identifier (0x02)
```

Currently, the hci_cs_disconnect function treats any non-zero status
as a command failure. This can be misleading because the connection is
indeed being terminated and the controller is confirming that is has no
knowledge of that connection handle. Meaning that the initial request of
disconnecting a device should be treated as done.

With this change we allow the function to proceed, following the success
path, which correctly calls `mgmt_device_disconnected` and ensures a
consistent state.

Link: https://github.com/bluez/bluez/issues/1226
Fixes: 182ee45da083 ("Bluetooth: hci_sync: Rework hci_suspend_notifier")
Signed-off-by: Ludovico de Nittis <ludovico.denittis@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>