Update copyright headers to use SPDX id's

src/avclan/target/avr-attiny3216/media_avr.c

@@ -1,27 +1,12 @@
-/*
-                        AVCLAN-Mockingboard
-    Copyright (C) 2015 Allen Hill <allenofthehills@gmail.com>
-
-    This program is free software: you can redistribute it and/or modify
-    it under the terms of the GNU General Public License as published by
-    the Free Software Foundation, either version 3 of the License, or
-    (at your option) any later version.
-
-    This program is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-    GNU General Public License for more details.
-
-    You should have received a copy of the GNU General Public License
-    along with this program.  If not, see <https://www.gnu.org/licenses/>.
-*/
+// Copyright (C) 2026 Allen Hill <allenofthehills@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 #include <avr/interrupt.h>
 #include <avr/io.h>
 #include <stdint.h>
 #include <util/atomic.h>
 
-#include "media_avr.h"   // mediacontrol_syncDuringMask (used by the bus guard)
+#include "media_avr.h" // mediacontrol_syncDuringMask (used by the bus guard)
 #include "mediacontrol.h"
 
 // F_CPU defined in timing_avr.h; the mic tick constants below are derived from