iio/gyro/bmg160: Use millidegrees for temperature scale

Standard unit for temperature is millidegrees Celcius, whereas this driver
was reporting in degrees. Fix the scale factor in the driver.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>