iio: imu: st_lsm6dsx: introduce update_fifo function pointer

Introduce update_fifo routine pointer in st_lsm6dsx_fifo_ops data
structure since we will need a different update FIFO configuration
callback adding support for lsm6ds0/lsm9ds1 imu device

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>