Followup to menu refactor. TMC warnings, sanity. (#12288)

- Followup fixes for menu refactor
- Fix TMC sanity checks, unused var warnings

Marlin/src/gcode/feature/trinamic/M906.cpp

@@ -37,7 +37,11 @@ void GcodeSuite::M906() {
   #define TMC_SET_CURRENT(Q) tmc_set_current(stepper##Q, value)
 
   bool report = true;
-  const uint8_t index = parser.byteval('I');
+
+  #if AXIS_IS_TMC(X) || AXIS_IS_TMC(X2) || AXIS_IS_TMC(Y) || AXIS_IS_TMC(Y2) || AXIS_IS_TMC(Z) || AXIS_IS_TMC(Z2) || AXIS_IS_TMC(Z3)
+    const uint8_t index = parser.byteval('I'); 
+  #endif
+
   LOOP_XYZE(i) if (uint16_t value = parser.intval(axis_codes[i])) {
     report = false;
     switch (i) {