power: twl4030-madc-battery: Convert to iio consumer.

Because of added iio error handling private data allocation was converted
to managed to simplify code.

Signed-off-by: Marek Belisko <marek@goldelico.com>
Reviewed-By: Sebastian Reichel <sre@debian.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>