staging: comedi: pcl726: tidy up pcl726_ao_insn()

For aesthetics, rename the function to help with greps.

The offset binary value from the core should be saved for read back.
Move the saving of the value in the private data so it occurs before
the value is possibly munged for bipolar outputs.

Use the comedi_offset_munge() helper to munge the offset binary value
to two's complement for bipolar outputs.

According to the November 2011 users manual, the write order must be
MSB them LSB. Update the comment.

Modify the register map defines to handle the channel offset calculation.

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>