Fix compiler warning in M145

Marlin/Marlin_main.cpp

@@ -5518,8 +5518,8 @@ inline void gcode_M140() {
    *   F<fan speed>
    */
   inline void gcode_M145() {
-    int8_t material = code_seen('S') ? (int8_t)code_value_int() : 0;
-    if (material < 0 || material >= COUNT(lcd_preheat_hotend_temp)) {
+    uint8_t material = code_seen('S') ? (uint8_t)code_value_int() : 0;
+    if (material >= COUNT(lcd_preheat_hotend_temp)) {
       SERIAL_ERROR_START;
       SERIAL_ERRORLNPGM(MSG_ERR_MATERIAL_INDEX);
     }