iio: imu: st_lsm6dsx: split fifo mode and fifo odr configuration

Separate fifo mode and max fifo sample rate configuration.
That change will be necessary to reuse st_lsm6dsx_set_fifo_mode()
routine and to 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>