Use const ref args in probe_pt

Marlin/Marlin_main.cpp

   //   - Raise to the BETWEEN height
   // - Return the probed Z position
   //
-  static float probe_pt(float x, float y, bool stow = true, int verbose_level = 1) {
+  static float probe_pt(const float &x, const float &y, bool stow = true, int verbose_level = 1) {
     #if ENABLED(DEBUG_LEVELING_FEATURE)
       if (DEBUGGING(LEVELING)) {
         SERIAL_ECHOPAIR(">>> probe_pt(", x);