iio: adc: ab8500-gpadc: Fix off by 10 to 3

Fix an off by three orders of magnitude error in the AB8500
GPADC driver. Luckily it showed up quite quickly when trying
to make use of it. The processed reads were returning
microvolts, microamperes and microcelsius instead of millivolts,
milliamperes and millicelsius as advertised.

Cc: stable@vger.kernel.org
Fixes: 07063bbf

 ("iio: adc: New driver for the AB8500 GPADC")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201224011700.1059659-1-linus.walleij@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>