staging: comedi: ni_stc.h: remove 'ao_unipolar' flag from ni_board_struct

This member of the boardinfo for the NI MIO drivers is used to indicate if
the ranges for the analog output subdevice (ao_range_table in the boardinfo)
includes any unipolar ranges. If it's not set, the ao_range_table only has
bipolar ranges.

The 'ao_unipolar' flag is checked when munging the ao data values from the
user so that the values for bipolar ranges are converted to 2's complement
values before they are written to the hardware.

The flag is also used when programming the analog output configuration on
non-M series boards for bipolar/unipolar and external reference operation.

Simplify the driver a bit by removing this boardinfo flag and just using
the comedi_range_is_bipolar() and comedi_range_is_external() helpers to
check the range directly.

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>