staging: comedi: me_daq: use comedi_range_is_bipolar() helper

Use the helper function to determine if the dac bipolar bit needs
to be set.

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>