staging: comedi: ni_mio_common: tidy up ni_ao_munge()

Use the bytes_per_sample() helper instead of 'sizeof(short)' to clarify
the length calculation.

Use comedi_offset_munge() to handle the unsigned to two's complement
munge of the data for bipolar ranges.

Tidy up the local variables.

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>