staging: comedi: das1800: remove hard coded 'sample_size'

Use the comedi_bytes_per_sample() helper to remove the hard coded sample_size
in suggest_transfer_size().

The helper function needs to comedi_subdevice pointer. Change the parameters
passed to suggest_transfer_size() and setup_dma() so that this pointer is
available. Rename these functions so they have namespace associated with 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>