thermal/core: Remove pointless test with the THERMAL_TRIPS_NONE macro

The THERMAL_TRIPS_NONE is equal to -1, it is pointless to do a
conversion in this function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Thara Gopinath <thara.gopinath@linaro.org>
Link: https://lore.kernel.org/r/20201214233811.485669-3-daniel.lezcano@linaro.org