extcon: adc-jack: Fix an error handling path in 'adc_jack_probe()'

In some error handling paths, a call to 'iio_channel_get()' is not balanced
by a corresponding call to 'iio_channel_release()'.

This can be achieved easily by using the devm_ variant of
'iio_channel_get()'.

This has the extra benefit to simplify the remove function.

Fixes: 19939860 ("extcon: adc_jack: adc-jack driver to support 3.5 pi or simliar devices")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>