staging: comedi: usbduxsigma: move urb unlink into usbduxsigma_detach()

For aesthetics, move the final force unlink of the urbs from
usbduxsigma_free_usb_buffers() to usbduxsigma_detach().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>