Fix serial port redirection (index ≠ port num) (#16687)

Marlin/src/core/serial.cpp

 static const char echomagic[]  PROGMEM = "echo:";
 
 #if NUM_SERIAL > 1
-  int8_t serial_port_index = SERIAL_PORT;
+  int8_t serial_port_index = 0;
 #endif
 
 void serialprintPGM(PGM_P str) {

Marlin/src/gcode/queue.cpp

   // Execute command if non-blank
   if (i) {
     parser.parse(cmd);
-    PORT_REDIRECT(SERIAL_PORT);
     gcode.process_parsed_command();
   }
   return true;
   #if NUM_SERIAL > 1
     const int16_t pn = port[index_r];
     if (pn < 0) return;
-    PORT_REDIRECT(pn);
+    PORT_REDIRECT(pn);                    // Reply to the serial port that sent the command
   #endif
   if (!send_ok[index_r]) return;
   SERIAL_ECHOPGM(MSG_OK);
  */
 void GCodeQueue::flush_and_request_resend() {
   #if NUM_SERIAL > 1
-    const int16_t p = port[index_r];
-    if (p < 0) return;
-    PORT_REDIRECT(p);
+    const int16_t pn = port[index_r];
+    if (pn < 0) return;
+    PORT_REDIRECT(pn);                    // Reply to the serial port that sent the command
   #endif
   SERIAL_FLUSH();
   SERIAL_ECHOPGM(MSG_RESEND);
   }
 }
 
-void GCodeQueue::gcode_line_error(PGM_P const err, const int8_t port) {
-  PORT_REDIRECT(port);
+void GCodeQueue::gcode_line_error(PGM_P const err, const int8_t pn) {
+  PORT_REDIRECT(pn);                      // Reply to the serial port that sent the command
   SERIAL_ERROR_START();
   serialprintPGM(err);
   SERIAL_ECHOLN(last_N);
-  while (read_serial(port) != -1);           // clear out the RX buffer
+  while (read_serial(pn) != -1);          // Clear out the RX buffer
   flush_and_request_resend();
-  serial_count[port] = 0;
+  serial_count[pn] = 0;
 }
 
 FORCE_INLINE bool is_M29(const char * const cmd) {  // matches "M29" & "M29 ", but not "M290", etc

Marlin/src/gcode/queue.h

    */
   static bool enqueue_one(const char* cmd);
 
-  static void gcode_line_error(PGM_P const err, const int8_t port);
+  static void gcode_line_error(PGM_P const err, const int8_t pn);
 
 };