iio: adc: ina2xx: Adhere to documented ABI, use Ohm instead of uOhm

According to the ABI documentation, the shunt resistor value should be
specificied in Ohm. As this is also used/documented for the MAX9611,
use the same for the INA2xx driver.

This poses an ABI break for anyone actually altering the shunt value
through the sysfs interface, it does not alter the default value nor
a value set from the devicetree.

Minor change: Fix comment, 1mA is 10^-3A.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>