iio: adc: sun4i-gpadc-iio: do not fail probing when no thermal DT node

Before this patch, forgetting to put a thermal-zones DT node would
result in the driver failing to probe.

It should be perfectly acceptable to have the driver probe even if no
thermal-zones DT is found. However, it shouldn't want to fail if the
thermal registering fail for any other reason (waiting for other drivers
for example) so check on ENODEV only.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>