power: supply: twl4030-charger: allocate iio by devm_iio_channel_get() and fix error path

Suggested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>