staging: comedi: adl_pci9118: use comedi_bytes_to_samples()

Remove the assumption of the sample size by using the comedi_bytes_to_samples()
helper function to convert the number of bytes to the number of samples.

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>