drivers/sensor: lsm6dso: Fix shub for multi-instance case

In case of multi-instance the driver tries to discover whether
there are devices attached to SDx/SCx sensorhub bus. If not it
just turns the shub_inited variable (inside data structure) to false
and skips doing any further sensorhub related action for that
particular LSM6DSO device instance, regardless the fact that the macro
CONFIG_LSM6DSO_SENSORHUB is enabled.

Moreover, the info found during the enumeration process for a
particular instance (number and types of attached devices) must be
saved inside the per-instance data structure, so that more than one
LSM6DSO device can be used as a sensorhub without interfering with
the others.

Signed-off-by: Armando Visconti <armando.visconti@st.com>