iio: ina2xx-adc: update the CALIB. register when RShunt changes

The user (or an init script) may setup RShunt via sysfs after the
driver was initialized, for instance based on the EEPROM contents
of a modular probe. The calibration register must be set accordingly.

Signed-off-by: Marc Titinger <marc.titinger@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>