thermal/drivers/qcom-spmi: Use devm_iio_channel_get

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: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>