staging: comedi: pcmmio: use core helpers to munge bipolar ai data

Use the comedi_range_is_bipolar() and comedi_offset_munge() helpers to
munge the bipolar analog input data.

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>