Tweaks to M915

Marlin/src/gcode/feature/trinamic/M911-M915.cpp

  */
 #if ENABLED(TMC_Z_CALIBRATION)
   void GcodeSuite::M915() {
-    uint16_t _rms = parser.seenval('S') ? parser.value_int() : CALIBRATION_CURRENT;
-    uint16_t _z = parser.seenval('Z') ? parser.value_int() : CALIBRATION_EXTRA_HEIGHT;
+    const uint16_t _rms = parser.seenval('S') ? parser.value_int() : CALIBRATION_CURRENT,
+                   _z = parser.seenval('Z') ? parser.value_linear_units() : CALIBRATION_EXTRA_HEIGHT;
 
     if (!axis_known_position[Z_AXIS]) {
       SERIAL_ECHOLNPGM("\nPlease home Z axis first");
     }
 
     #if Z_IS_TRINAMIC
-      uint16_t Z_current_1 = stepperZ.getCurrent();
+      const uint16_t Z_current_1 = stepperZ.getCurrent();
       stepperZ.setCurrent(_rms, R_SENSE, HOLD_MULTIPLIER);
     #endif
     #if Z2_IS_TRINAMIC
-      uint16_t Z2_current_1 = stepperZ2.getCurrent();
+      const uint16_t Z2_current_1 = stepperZ2.getCurrent();
       stepperZ2.setCurrent(_rms, R_SENSE, HOLD_MULTIPLIER);
     #endif