staging: comedi: dt282x: fix dt282x_ao_insn_write()

The (*insn_write) functions are expected to write 'insn->n' samples to the
hardware. Fix this function so it works like the comedi core expects.

The comedi core sanity checks that the samples are within the 'maxdata' range
of the subdevice before calling the (*insn_write) so this function does not
need to mask each sample by 's->maxdata'.

Also, the wrong '*2scomp' flag in the private data was being checked to see
if the sample needs to be munged before being written. Fix this.

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>