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>