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

Introduce some helper functions to check if a given 'range' index
to a comedi_subdevice 'range_table' is a bipolar or unipolar range.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>