Do not allow configuring null sensor gain since it will force to 0
device outputs

Fixes: c8d4066c7246 ("iio: imu: st_lsm6dsx: remove invalid gain value for LSM9DS1")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>