Further clean up for serial over run issue during long operations

Marlin/G26_Mesh_Validation_Tool.cpp

@@ -274,6 +274,9 @@
                                     // action to give the user a more responsive 'Stop'.
           set_destination_from_current();
           idle();
+          MYSERIAL.flush();  // G26 takes a long time to complete.   PronterFace can
+                             // over run the serial character buffer with M105's without
+                             // this fix
         }
 
         wait_for_release();
@@ -822,7 +825,9 @@
           //}
 
           print_line_from_here_to_there(rx, ry, g26_layer_height, xe, ye, g26_layer_height);
-
+          MYSERIAL.flush();  // G26 takes a long time to complete.   PronterFace can
+                             // over run the serial character buffer with M105's without
+                             // this fix
         }
         if (look_for_lines_to_connect())
           goto LEAVE;