this patch indroduces an overflow bug:

bb859ef7 drv bugfix according to ZIO offset is uV

hwval is 32bit, but since user values are now bigger (uV) the
multiplication can overflow. We use 64bit to avoid it.
Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Signed-off-by: Milosz Malczak <milosz.malczak@cern.ch>