iio: imu: st_lsm6dsx: enable drdy-mask if available

Enable drdy mask if available in order to mark invalid samples during
sensor bootstrap phase

Tested-by: Mario Tesi <mario.tesi@st.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>