thermal: stm32: Improve temperature computing

Change the way of computing to avoid rounds by 1 or 2 degrees.
Also simplify the sampling time management that is hard-coded
to maximum value during probe.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-6-p.paillet@st.com