Updates Stopwatch class to use internal state enum

Marlin/stopwatch.cpp

   #endif
 
   if (this->isRunning() || this->isPaused()) {
-    this->state = STOPWATCH_STOPPED;
+    this->state = STOPPED;
     this->stopTimestamp = millis();
     return true;
   }
   #endif
 
   if (this->isRunning()) {
-    this->state = STOPWATCH_PAUSED;
+    this->state = PAUSED;
     this->stopTimestamp = millis();
     return true;
   }
     if (this->isPaused()) this->accumulator = this->duration();
     else this->reset();
 
-    this->state = STOPWATCH_RUNNING;
+    this->state = RUNNING;
     this->startTimestamp = millis();
     return true;
   }
     Stopwatch::debug(PSTR("reset"));
   #endif
 
-  this->state = STOPWATCH_STOPPED;
+  this->state = STOPPED;
   this->startTimestamp = 0;
   this->stopTimestamp = 0;
   this->accumulator = 0;
 }
 
 bool Stopwatch::isRunning() {
-  return (this->state == STOPWATCH_RUNNING) ? true : false;
+  return (this->state == RUNNING) ? true : false;
 }
 
 bool Stopwatch::isPaused() {
-  return (this->state == STOPWATCH_PAUSED) ? true : false;
+  return (this->state == PAUSED) ? true : false;
 }
 
 millis_t Stopwatch::duration() {

Marlin/stopwatch.h

 // Print debug messages with M111 S2 (Uses 156 bytes of PROGMEM)
 //#define DEBUG_STOPWATCH
 
-enum StopwatchState {
-  STOPWATCH_STOPPED,
-  STOPWATCH_RUNNING,
-  STOPWATCH_PAUSED
-};
-
 /**
  * @brief Stopwatch class
  * @details This class acts as a timer proving stopwatch functionality including
  */
 class Stopwatch {
   private:
-    StopwatchState state;
+    enum State {
+      STOPPED,
+      RUNNING,
+      PAUSED
+    };
+
+    Stopwatch::State state;
     millis_t accumulator;
     millis_t startTimestamp;
     millis_t stopTimestamp;