iio: imu: st_lsm6dsx: fix FIFO threshold mask for LSM6DSM/LSM6DSL

According to the latest datasheet, LSM6DSM and LSM6DSL imu sensors
export GENMASK(10, 0) for FIFO threshold mask definition. Despite
that is not an actual issue since write on fifo_th register is protected
by max_fifo_size value, fix fifo_th and fifo_diff register definitions

Fixes: 290a6ce1 ("iio: imu: add support to lsm6dsx driver")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>