Fix G81 for loop

Firmware/Marlin_main.cpp

@@ -5623,7 +5623,7 @@ if(eSoundMode!=e_SOUND_MODE_SILENT)
                 SERIAL_PROTOCOLPGM("\nZ search height: ");
                 SERIAL_PROTOCOL(MESH_HOME_Z_SEARCH);
                 SERIAL_PROTOCOLLNPGM("\nMeasured points:");
-                for (int16_t y = MESH_NUM_Y_POINTS-1; y >= 0; y--) {
+                for (uint8_t y = MESH_NUM_Y_POINTS; y-- > 0;) {
                     for (uint8_t x = 0; x < MESH_NUM_X_POINTS; x++) {
                         SERIAL_PROTOCOLPGM("  ");
                         SERIAL_PROTOCOL_F(mbl.z_values[y][x], 5);