Update copyright headers to use SPDX id's

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

@@ -1,26 +1,9 @@
+// copyright (C) 2006 Marcin Slonicki <marcin@softservice.com.pl>
+// copyright (C) 2007 Louis Frigon
+// Copyright (C) 2015 Allen Hill <allenofthehills@gmail.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 /*
-                        AVCLAN-Mockingboard
-    Copyright (C) 2015 Allen Hill <allenofthehills@gmail.com>
-
-    Portions of the following source code are based on code that is
-    copyright (C) 2006 Marcin Slonicki <marcin@softservice.com.pl>
-    copyright (C) 2007 Louis Frigon
-
-    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/>.
-
---------------------------------------------------------------------------------------
-
                                 AVC LAN Theory
 
   The AVC LAN bus is an implementation of the IEBus (mode 1) which is a
@@ -60,8 +43,6 @@
     - For acknowledge bits, the receiver extends the logical '0' of the sync
       period to the length of a normal bit `0`. Hence, a NAK (bit `1`) is
       literally the absence of an ACK.
-
---------------------------------------------------------------------------------------
 */
 
 #include <avr/interrupt.h>