staging: comedi: core: introduce comedi_chan_range_is_{bi,uni}polar()

Introduce two helper functions to check if a subdevice range_table_list
for a given channel/range is bipolar or unipolar.

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>