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

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

`usbduxfast_detach()` is the Comedi "detach" handler for the usbduxfast
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
`usbduxfast_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>