iio: ina2xx: re-instate a sysfs show/store for the shunt resistor value

Different probe modules use different resistor values. The front-end
application may read a probe ID (from eeprom) and set the shunt value
accordingly.

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