Apply SERIAL_FLUSH macro

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

           const float measured_z = probe_pt(rawx, rawy, stow_probe, g29_verbose_level); // TODO: Needs error handling
           z_values[location.x_index][location.y_index] = measured_z;
         }
-        MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+        SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
       } while (location.x_index >= 0 && --max_iterations);
 
       STOW_PROBE();
           SERIAL_PROTOCOL_F(z_values[location.x_index][location.y_index], 6);
           SERIAL_EOL();
         }
-        MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+        SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
       } while (location.x_index >= 0 && location.y_index >= 0);
 
       if (do_ubl_mesh_map) display_map(g29_map_type);  // show user where we're probing
             do_blocking_move_to_z(h_offset + new_z); // Move the nozzle as the point is edited
           #endif
           idle();
-          MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+          SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
         } while (!is_lcd_clicked());
 
         if (!lcd_map_control) lcd_return_to_status();

Marlin/src/gcode/bedlevel/G26.cpp

             SERIAL_EOL();
           }
           idle();
-          MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+          SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
         }
     #if ENABLED(ULTRA_LCD)
       }
       SERIAL_EOL();
     }
     idle();
-    MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+    SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
   }
 
   #if ENABLED(ULTRA_LCD)
 
 
         print_line_from_here_to_there(rx, ry, g26_layer_height, xe, ye, g26_layer_height);
-        MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+        SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
       }
       if (look_for_lines_to_connect())
         goto LEAVE;
     }
-    MYSERIAL0.flush(); // Prevent host M105 buffer overrun.
+    SERIAL_FLUSH(); // Prevent host M105 buffer overrun.
   } while (--g26_repeats && location.x_index >= 0 && location.y_index >= 0);
 
   LEAVE: