iio: adc: max9611: Fix attribute measure unit

The power and current "shunt-resistor" attribute's 'show' function
displays the resistor value in milli-Ohms, while the ABI description
specifies it should be displayed in Ohms. Fix it.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>