Minor change to fan kick

Marlin/planner.cpp

@@ -429,11 +429,12 @@ void check_axes_activity() {
     #ifdef FAN_KICKSTART_TIME
       static millis_t fan_kick_end;
       if (tail_fan_speed) {
+        millis_t ms = millis();
         if (fan_kick_end == 0) {
           // Just starting up fan - run at full power.
-          fan_kick_end = millis() + FAN_KICKSTART_TIME;
+          fan_kick_end = ms + FAN_KICKSTART_TIME;
           tail_fan_speed = 255;
-        } else if (fan_kick_end > millis())
+        } else if (fan_kick_end > ms)
           // Fan still spinning up.
           tail_fan_speed = 255;
         } else {