iio: imu: st_lsm6dsx: convert max_fifo_size in FIFO sample size

Express max fifo depth in ST_LSM6DSX_SAMPLE_SIZE instead of in bytes.
That change will be necessary to properly support more devices
in st_lsm6dsx driver

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>