thermal-generic-adc: Silent error message for EPROBE_DEFER

If devm_iio_channel_get() or devm_thermal_zone_of_sensor_register()
fail with EPROBE_DEFER, we shouldn't print an error message, as the
device will be probed again later.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20190910075907.132200-1-hsinyi@chromium.org