openocd: src: fix incorrect GPL license tag

Use the standard SPDX tag, where it was incorrectly applied.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: Iaec63abc6e0a38e5b0ae0ea7f5ecee7ca007bbbd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7065
Tested-by: jenkins

src/flash/nor/xmc1xxx.c

@@ -1,9 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
 /*
  * XMC1000 flash driver
  *
  * Copyright (c) 2016 Andreas Färber
- *
- * License: GPL-2.0+
  */
 
 #ifdef HAVE_CONFIG_H