The library was not converting back the offset from the DAC raw value to
uV as expected by the API. This bug was masked by a previously fixed bug.

   143b17 lib: bugfix do not override value on write

Beware that the offset gets approximated to a 16bit number withing the
range [-5V, 4.999V]. This means that most of the time the read back is
not exactly what was configured but its hardware approximation (~152uV
steps).
Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Reported-by: Nicolas Magnin <nicolas.magnin@cern.ch>