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

`usbdux_detach()` is the Comedi "detach" handler for the usbdux 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()` 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>