staging: comedi: ni_mio_common: fix ni_ao_insn_write_671x()

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

Also, use the comedi_offset_munge() helper to convert the comedi unsigned
values into the two's complement values that the hardware needs.

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>