staging: comedi: s626: use comedi_buf_write_samples()

Use comedi_buf_write_samples() instead of cfc_write_to_buffer() to add the
single 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>