iio:chemical:bme680: Fix, report temperature in millidegrees

The standard unit for temperature is millidegrees Celcius. Adapt the
driver to report in millidegrees instead of degrees.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Fixes: 1b3bd859 ("iio: chemical: Add support for Bosch BME680 sensor");
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>