iio: adc: ina2xx: Use LSB specifier instead of divider in config

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>