While the config uses the physical value corresponding to the LSB
for both the power and the bus voltage register, the shunt voltage is
specified as parts of 1 mV. Use the LSB physical value for all registers.

No functional change.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>