staging: comedi: rtd520: remove private data 'chan_is_bipolar' member

Currently this driver uses a bitmap in the private data to keep track of
the unipolar/bipolar range for each channel. This is needed to determine
if the data needs to be munged for bipolar samples.

Remove this member from the private data and use the comedi core helper
function comedi_range_is_bipolar() to determine if the data needs to be
munged.

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>