iio: imu: st_lsm6dsx: export max num of slave devices in st_lsm6dsx_shub_settings

Export max number of slave devices supported by the i2c master
controller in st_lsm6dsx_shub_settings data structure.
This is a preliminary patch to support i2c master controller
on lsm6dsm sensors

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