Fix an issue found in code review

Unfortunately this increases flash usages by 212 bytes (same usage as in current release) It seems it is most memory effcient to use int16_t (int). int8_t requires more memory.

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 (uint8_t y = MESH_NUM_Y_POINTS-1; y >= 0; y--) {
+                for (int16_t y = MESH_NUM_Y_POINTS-1; y >= 0; y--) {
                     for (uint8_t x = 0; x < MESH_NUM_X_POINTS; x++) {
                         SERIAL_PROTOCOLPGM("  ");
                         SERIAL_PROTOCOL_F(mbl.z_values[y][x], 5);