staging: comedi: ni_usb6501: Call mutex_destroy() on private mutex

`ni6501_detach()` is the Comedi "detach" handler for the ni_usb6501
driver.  It is called when the private data for the device is about to
be freed.  The private data contains a mutex `devpriv->mut` that was
initialized when the private data was allocated.  Call `mutex_destroy()`
to mark it as invalid.

Also remove the calls to `mutex_lock()` and `mutex_unlock()` from
`ni6501_detach()`.  The only other locks of the mutex are by some of the
Comedi instruction handlers that cannot contend with the "detach"
handler for this mutex.

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