thermal/core: Use clamp() helper in the stepwise governor

The code is actually clampling the next cooling device state using the
lowest and highest states of the thermal instance.

That code can be replaced by the clamp() macro which does exactly the
same. It results in a simpler routine to read.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220629151012.3115773-1-daniel.lezcano@linaro.org


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>