fix thermal protection state machine target temp. V2

If the target temperature is changed then it would always stay in the reset state.
Thanks to @tonokip.

Replaces #2119

Marlin/temperature.cpp

       case TRReset:
         *timer = 0;
         *state = TRInactive;
-        break;
       // Inactive state waits for a target temperature to be set
       case TRInactive:
         if (target_temperature > 0) {