Bluetooth: hci_serdev: Only unregister device if it was registered

We should not call hci_unregister_dev if the device was not
successfully registered.

Fixes: c34dc3bf

 ("Bluetooth: hci_serdev: Introduce hci_uart_unregister_device()")
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>