Fix M420 C for UBL

Marlin/src/feature/bedlevel/ubl/ubl.h

     static void reset();
     static void invalidate();
     static void set_all_mesh_points_to_value(const float value);
-    static void adjust_mesh_to_mean(const float value);
+    static void adjust_mesh_to_mean(const bool cflag, const float value);
     static bool sanity_check();
 
     static void G29() _O0;                          // O0 for no optimization

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

           #endif
           break;
 
-        case 5: adjust_mesh_to_mean(g29_constant); break;
+        case 5: adjust_mesh_to_mean(g29_c_flag, g29_constant); break;
 
         case 6: shift_mesh_height(); break;
       }
     return;
   }
 
-  void unified_bed_leveling::adjust_mesh_to_mean(const float value) {
+  void unified_bed_leveling::adjust_mesh_to_mean(const bool cflag, const float value) {
     float sum = 0.0;
     int n = 0;
     for (uint8_t x = 0; x < GRID_MAX_POINTS_X; x++)
     SERIAL_ECHO_F(sigma, 6);
     SERIAL_EOL();
 
-    if (g29_c_flag)
+    if (cflag)
       for (uint8_t x = 0; x < GRID_MAX_POINTS_X; x++)
         for (uint8_t y = 0; y < GRID_MAX_POINTS_Y; y++)
           if (!isnan(z_values[x][y]))
       SERIAL_EOL();
     #endif
 
-    adjust_mesh_to_mean(g29_constant);
+    adjust_mesh_to_mean(g29_c_flag, g29_constant);
 
     #if HAS_BED_PROBE
       SERIAL_PROTOCOLPGM("zprobe_zoffset: ");

Marlin/src/gcode/bedlevel/M420.cpp

       #if ENABLED(AUTO_BED_LEVELING_UBL)
 
         set_bed_leveling_enabled(false);
-        ubl.adjust_mesh_to_mean(cval);
+        ubl.adjust_mesh_to_mean(true, cval);
 
       #else