staging: comedi: ni_at_ao: tidy up atao_ao_winsn()

For aesthetics, rename this function to help with greps.

For channel 0, register group 1 only needs to be selected before
writing the values and restored to group 0 when the writes are
finished. Move the code out of the for () loop.

Use the comedi_ffset_munge() helper to handle the offset binary to
two's complement munging.

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>