power: supply: lp8788: Fix an error handling path in 'lp8788_charger_probe()'

In the probe function, in case of error, resources allocated in
'lp8788_setup_adc_channel()' must be released.

This can be achieved easily by using the devm_ variant of
'iio_channel_get()'.
This has the extra benefit to simplify the remove function and to axe the
'lp8788_release_adc_channel()' function which is now useless.

Fixes: 98a27664 ("power_supply: Add new lp8788 charger driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>