hwmon: (ina2xx) Use shunt voltage to calculate current

Since the shunt voltage and the current register report the same values
when the chip is calibrated, we can calculate the current directly
from the shunt voltage without relying on chip calibration.

With this change, the current register is no longer accessed. Its
register address is only used to indicate if reading or writing
current or shunt voltage is desired when accessing registers.

Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>