thermal: stm32: Rework sensor mode management

Be sure get_temp returns an error while disabling or enabling the device.
Set THERMAL_DEVICE_ENABLED state at the end of power on function.
Set THERMAL_DEVICE_DISABLED state at the beginning of power off function.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200110101605.24984-3-p.paillet@st.com