The iio_channel_get() function has now its devm_ version.

Use it and remove all the rollback code for iio_channel_release() as well
as the .remove ops.

[Compiled tested only]
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: NEduardo Valentin <edubezval@gmail.com>