Fix a compile warning, etc. (#12955)

Marlin/src/gcode/sdcard/M20-M30_M32-M34_M524_M928.cpp

@@ -179,13 +179,9 @@ void GcodeSuite::M28() {
 
   #if ENABLED(FAST_FILE_TRANSFER)
 
-    const int16_t port =
-      #if NUM_SERIAL > 1
-        command_queue_port[cmd_queue_index_r]
-      #else
-        0
-      #endif
-    ;
+    #if NUM_SERIAL > 1
+      const int16_t port = command_queue_port[cmd_queue_index_r];
+    #endif
 
     bool binary_mode = false;
     char *p = parser.string_arg;