thermal: stm32: Fix icifr register name

Fix a mistake with the ICIFR register name.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-2-p.paillet@st.com