iio: imu: st_lsm6dsx: correct ODR in header

Fix wrongly stated 13 Hz ODR for accelerometers, the correct ODR is 12.5 Hz

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>