staging: comedi: ni_mio_common: fix ni_ao_insn_write()

Comedi (*insn_write) functions are expected to write insn->n values
to the hardware. Fix this function to work like the core expects.

Also, don't rely on the return value of ni_ao_comfig_chanlist() to
determine if the values need converted to two's complement before
writing to the hardware. Use the comedi_range_is_bipolar() and
comedi_offset_munge() helpers to clarify the code.

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>