staging: comedi: das16m1: convert munge_sample_array() into a subdevice (*munge)

For aesthetics, convert this function into a subdevice (*munge) function and let
the comedi core handle the munging.

Add a comment about why the data needs to be munged.

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>