Everything mentionned here:
https://lkml.org/lkml/2016/4/20/850
This driver was added before the devm_iio_channel_get() function version was
merged. The sensor should be released before the iio channel, thus we had to
use the non-devm version of thermal_zone_of_sensor_register().
Now the devm_iio_channel_get() is available, do the corresponding change in
this driver and remove gadc_thermal_remove().