staging: comedi: cb_pcidas: introduce cb_pcidas_ao_load_fifo()

Introduce a helper function to load the analog output FIFO.

This fixes a bug in cb_pcidas_ao_inttrig(). That function was writing
'num_bytes' data values to the FIFO instead of 'num_points'. This results
in twice the number of data values getting written to the FIFO.

Use comedi_buf_read_samples() 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>