iio: imu: st_lsm6dsx: explicitly define odr table size

Introduce odr_len in st_lsm6dsx_odr_table_entry data structure in order
to explicitly define odr table size and support devices with different
odr table map

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>