staging: comedi: hwdrv_apci3120: use comedi_buf_write_samples()

Use comedi_buf_write_samples() to add the samples to the async buffer.
That function will handle the '* sizeof(foo)' calculation to determine
the number of bytes to add.

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>