staging: comedi: usbduxsigma: Call mutex_destroy() on private mutex

`usbduxsigma_detach()` is the Comedi "detach" handler for the
usbduxsigma driver.  When it is called, the private data for the device
is about to be freed.  The private date contains a mutex `devpriv->mut`
that was initialized when the private data was allocated.  Call
`mutex_destroy()` to mark it as invalid.

The calls to `mutex_lock()` and `mutex_unlock()` in
`usbduxsigma_detach()` are probably not required, especially as the
mutex is about to be destroyed, but leave them alone for now.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/usbduxsigma.c

@@ -1577,6 +1577,8 @@ static void usbduxsigma_detach(struct comedi_device *dev)
 	usbduxsigma_free_usb_buffers(dev);
 
 	mutex_unlock(&devpriv->mut);
+
+	mutex_destroy(&devpriv->mut);
 }
 
 static struct comedi_driver usbduxsigma_driver = {