staging: comedi: pcl726: remove 'bipolar' from the private data

These flags are set in the private data during the attach to indicate
if the range for each channel is bipolar or unipolar. Use the helper
function conedi_chan_range_is_bipolar() to determine this by checking
the range_table_list directly.

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>