staging: comedi: dmm32at: use comedi_buf_write_samples()

For aesthetics, use comedi_buf_write_samples() to add the sample to 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>