power: supply: generic-adc-battery: fix unit scaling

power-supply properties are reported in µV, µA and µW.
The IIO API provides mV, mA, mW, so the values need to
be multiplied by 1000.

Fixes: e60fea79 ("power: battery: Generic battery driver using IIO")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>