staging: comedi: das08: clarify sign-magnitude conversion

For DAS08/JR/16 and DAS08/JR/AO-16 boards, the 16-bit analog input
readings are assumed to be in a sign-magnitude format and need
converting to the COMEDI unsigned sample format.  The expressions to do
the conversion look a little messy.  Use a local variable `magnitude` to
make it easier to follow.

Also, there seems to be some discrepancy between the manual for these
boards and the COMEDI code.  The manual implies that 0 is full-scale
negative and 65535 is full-scale positive.  However, the COMEDI code has
used the sign-magnitude conversion for these boards since these two
boards were initially supported by a patch from an external contributor
to the COMEDI project back in 2001.  Assume the code is correct for now,
but add a comment to mention the discrepancy.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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