staging: comedi: ni_labpc: introduce labpc_range_is_unipolar()

Introduce a helper function to determine if a range selection is
bipolar or unipolar.

This allows removing the static data 'labpc_range_is_unipolar' and
'labpc_plus_is_unipolar' as well as the boardinfo 'ai_range_is_unipolar'.

For consistency, also use the helper in labpc_ao_winsn() when
checking the range.

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