staging: comedi: usbduxfast: use comedi_buf_write_samples()

Use comedi_buf_write_samples() to add the samples to the async buffer.
That function will determining the number of bytes to add based on the
bytes_per_sample().

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>