Make M303 silent when "S0" is sent

If S is left out perhaps it should be silent, but check the spec for
this M code.

Marlin/Configuration.h

@@ -298,7 +298,7 @@ const bool Z_ENDSTOPS_INVERTING = true; // set to true to invert the logic of th
 // #define EXTRUDER_OFFSET_X {0.0, 20.00} // (in mm) for each extruder, offset of the hotend on the X axis
 // #define EXTRUDER_OFFSET_Y {0.0, 5.00}  // (in mm) for each extruder, offset of the hotend on the Y axis
 
-// The speed change that does not require acceleration (i.e. the software might assume it can be done instanteneously)
+// The speed change that does not require acceleration (i.e. the software might assume it can be done instantaneously)
 #define DEFAULT_XYJERK                20.0    // (mm/sec)
 #define DEFAULT_ZJERK                 0.4     // (mm/sec)
 #define DEFAULT_EJERK                 5.0    // (mm/sec)

Marlin/Marlin_main.cpp

@@ -1449,14 +1449,19 @@ void process_commands()
     #if defined(LARGE_FLASH) && LARGE_FLASH == true && defined(BEEPER) && BEEPER > -1
     case 300: // M300
     {
-      int beepS = 1;
+      int beepS = code_seen('S') ? code_value() : 110;
-      int beepP = 1000;
+      int beepP = code_seen('P') ? code_value() : 1000;
-      if(code_seen('S')) beepS = code_value();
+      if (beepS > 0)
-      if(code_seen('P')) beepP = code_value();
+      {
         tone(BEEPER, beepS);
         delay(beepP);
         noTone(BEEPER);
       }
+      else
+      {
+        delay(beepP);
+      }
+    }
     break;
     #endif // M300