thermal: stm32: Disable interrupts at probe

In case of CPU reset, the interrupts could be enabled at boot time.
Disable interrupts and clear flags.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-4-p.paillet@st.com