staging: comedi: dt282x: introduce dt282x_ao_setup_dma()

Introduce a helper function to read the analog output samples from the
async buffer and prep the DMA.

Use comedi_buf_read_samples() to read the analog output samples from
the async buffer.

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>