staging: comedi: pcl816: use common function to setup dma

THe pcl816_ai_setup_dma() and pcl816_ai_setup_next_dma() functions are similar
other than the buffer switch and the inclusion of the "unread_samples" in
pcl818_ai_setup_next_dma() when calculating the dma size.

Merge these two functions by initializing the 'dma->cur_dma' in the callers
and passing '0' for the "unread_samples" when first starting the DMA.

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>