iio: imu: st_lsm6dsx: specify slave odr in slv_odr

Introduce slv_odr in ext_info data structure in order to distinguish
between sensor hub trigger (accel sensor) odr and i2c slave odr and
properly compute samples in 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>