staging: comedi: das800: 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.

The core will add the COMEDI_CB_BLOCK event when data is written to the
async buffer. Remove the unnecessary event from the driver.

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>