iio: imu: st_lsm6dsx: fix read misalignment on untagged FIFO

st_lsm6dsx suffers of a read misalignment on untagged FIFO when
all 3 supported sensors (accel, gyro and ext device) are running
at different ODRs (the use-case is reported in the LSM6DSM Application
Note at pag 100).
Fix the issue taking into account decimation factor reading the FIFO
pattern.

Fixes: e485e2a2 ("iio: imu: st_lsm6dsx: enable sensor-hub support for lsm6dsm")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>