thermal/drivers/generic-iio-adc: Switch tz request to devm version

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().

[Compiled tested only]

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