thermal: stm32: read factory settings inside stm_thermal_prepare

Calling stm_thermal_read_factory_settings before clocking
internal peripheral causes bad register values and makes
temperature computation wrong.

Calling stm_thermal_read_factory_settings inside
stm_thermal_prepare fixes this problem as internal
peripheral is well clocked at this stage.

Fixes: 1d693155 ("thermal: add stm32 thermal driver")
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: David Hernandez Sanchez <david.hernandezsanchez@st.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>