Bluetooth: hci_sync: Fix queuing commands when HCI_UNREGISTER is set

hci_cmd_sync_queue shall return an error if HCI_UNREGISTER flag has
been set as that means hci_unregister_dev has been called so it will
likely cause a uaf after the timeout as the hdev will be freed.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>