staging: comedi: usbduxsigma: tidy up tidy_up()

Rename the function so it has namespace associated with the driver.

For aesthetic reasons, move the function so it's near the one that
allocates the usb buffers.

Tidy up the function so it frees the buffers in the opposite order
they were allocated. Also remove the = NULL for all the buffers,
the driver is being detached so the comedi core will kfree the
private data.

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