staging: comedi: rtd520: remove broken USE_DMA code

It appears the dma code in this driver is seriously broken.
Enabling USE_DMA causes the driver to not even compile.

Just remove all the dma code for now.
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>