thermal: step_wise: cdev only needs update on a new target state

The cooling device only needs update on a new target state.  Since we
already check old target in thermal_zone_trip_update(), we can do one
more check to see if it's a new target state.  If not, we can reasonably
save some uncecesary code execution.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>