drivers/sensor: st: fix ASSERT in fetch API

Many drivers use a wrong __ASSERT in the fetch API, where they just
use SENSOR_CHAN_ALL, while it is more common fetching the specific
sensor channel (i.e. SENSOR_CHAN_PRESS, SENSOR_CHAN_ACCEL_XYZ, ...).

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