Fix compile of MMU2 with S-mode disabled (#19584)

Marlin/src/feature/mmu2/mmu2.cpp

       #endif
 
       if (rx_ok()) {
-        // response to C0 mmu command in PRUSA_MMU2_S_MODE
+        // Response to C0 mmu command in PRUSA_MMU2_S_MODE
         bool can_reset = true;
-        if (ENABLED(PRUSA_MMU2_S_MODE) && last_cmd == MMU_CMD_C0) {
-          if (!mmu2s_triggered) {
+        #if ENABLED(PRUSA_MMU2_S_MODE)
+          if (!mmu2s_triggered && last_cmd == MMU_CMD_C0) {
             can_reset = false;
             // MMU ok received but filament sensor not triggered, retrying...
             DEBUG_ECHOLNPGM("MMU => 'ok' (filament not present in gears)");
             DEBUG_ECHOLNPGM("MMU <= 'C0' (keep trying)");
             MMU2_COMMAND("C0");
           }
-        }
+        #endif
         if (can_reset) {
           DEBUG_ECHOLNPGM("MMU => 'ok'");
           ready = true;