staging: comedi: pcl812: factor out dma setup to helper functions

To assist in cleaning up the the rest of the driver, factor the initial
dma setup and the next dma buffer setup into helper functions.

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>