staging: comedi: adq12b: remove 'unipolar' from private data

This member of the private data is only used during the attach to
determine which range table to use for the analog inputs. Remove
the member and use the comedi_devconfig option 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>