Change set_z() parameters to uin8_t

Saves 24 bytes of flash

Firmware/mesh_bed_calibration.cpp

@@ -2872,14 +2872,14 @@ bool sample_mesh_and_store_reference()
 		}
         mbl.set_z(0, 0, current_position[Z_AXIS]);
     }
-    for (int8_t mesh_point = 1; mesh_point != MESH_MEAS_NUM_X_POINTS * MESH_MEAS_NUM_Y_POINTS; ++ mesh_point) {
+    for (uint8_t mesh_point = 1; mesh_point != MESH_MEAS_NUM_X_POINTS * MESH_MEAS_NUM_Y_POINTS; ++ mesh_point) {
         // Don't let the manage_inactivity() function remove power from the motors.
         refresh_cmd_timeout();
         // Print the decrasing ID of the measurement point.
         current_position[Z_AXIS] = MESH_HOME_Z_SEARCH;
         go_to_current(homing_feedrate[Z_AXIS]/60);
-		int8_t ix = mesh_point % MESH_MEAS_NUM_X_POINTS;
-		int8_t iy = mesh_point / MESH_MEAS_NUM_X_POINTS;
+		uint8_t ix = mesh_point % MESH_MEAS_NUM_X_POINTS;
+		uint8_t iy = mesh_point / MESH_MEAS_NUM_X_POINTS;
 		if (iy & 1) ix = (MESH_MEAS_NUM_X_POINTS - 1) - ix; // Zig zag
 		current_position[X_AXIS] = BED_X(ix, MESH_MEAS_NUM_X_POINTS);
 		current_position[Y_AXIS] = BED_Y(iy, MESH_MEAS_NUM_Y_POINTS);

Firmware/mesh_bed_leveling.h

@@ -29,7 +29,7 @@ public:
     // Otherwise a correction matrix is pulled from the EEPROM if available.
     static void get_meas_xy(int ix, int iy, float &x, float &y, bool use_default);
     
-    void set_z(int ix, int iy, float z) { z_values[iy][ix] = z; }
+    void set_z(uint8_t ix, uint8_t iy, float z) { z_values[iy][ix] = z; }
     
     int select_x_index(float x) {
         int i = 1;