iio: humidity: hdc3020: fix temperature offset

The temperature offset should be negative according to the datasheet.
Adding a minus to the existing offset results in correct temperature
calculations.

Fixes: c9180b8e

 ("iio: humidity: Add driver for ti HDC302x humidity sensors")
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Dimitri Fedrau <dima.fedrau@gmail.com>
Link: https://lore.kernel.org/r/20240126135226.3977904-1-dima.fedrau@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>