staging: comedi: cb_pcidas: introduce cb_pcidas64_ao_fill_buffer()

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

This fixes a bug in load_ao_dma_buffer(). That function was decrementing
the 'ao_count' by the number of bytes, not the number of samples, that
were read from the async buffer.

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>