Merge pull request #2193 from thinkyhead/ignore_unrecognized_commands

Ignore unknown commands if they start with G, M, or T

Marlin/Marlin_main.cpp

       case 92: // G92
         gcode_G92();
         break;
-
-      default: code_is_good = false;
     }
     break;
 
         gcode_M109();
         break;
 
-      case 110: break; // M110: Set line number - don't show "unknown command"
-
       #if HAS_TEMP_BED
         case 190: // M190: Wait for bed heater to reach target
           gcode_M190();
       case 999: // M999: Restart after being Stopped
         gcode_M999();
         break;
-
-      default: code_is_good = false;
     }
     break;
 
     case 'T':
       gcode_T(codenum);
     break;
+
+    default: code_is_good = false;
   }
 
 ExitUnknownCommand: