power: supply: ab8500: Use iio_read_channel_processed_scale()

Instead of rescaling current or voltage channels after the fact, use the
dedicated scaling API. This should reduce any inaccuracies resulting from
the scaling.

This is also slightly more efficient as it saves a function call and a
multiplication.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/5668d73b92eb6318c7f094a9a8fa914c909485ca.1719037737.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>