staging: comedi: gsc_hpdi: tidy up setup_dma_descriptors()

For aesthetics, rename this function so it has namespace associated
with the driver and move it so that it is not in the middle of the
async command support code.

Tidy up the function a bit.

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>