Bluetooth: Skip the shutdown routine if the interface is not up

Most likely, the shutdown routine requires the interface to be up.
This is the case for BTUSB_INTEL: the routine tries to send a command
to the interface, but since this one is down, it fails and exits once
HCI_INIT_TIMEOUT has expired.

Signed-off-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 4.0.x