ASoC: switch audio drivers to SPDX identifier

Switch audio drivers to SPDX identifier on msm-4.19.

Change-Id: Ic3f0230a516db251b8d81e8a7e73dbc04d66fe1b
Signed-off-by: Meng Wang <mengw@codeaurora.org>

asoc/codecs/wcd9xxx-slimslave.c

@@ -1,13 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2012-2017, The Linux Foundation. All rights reserved.
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 and
- * only version 2 as published by the Free Software Foundation.
- *
- * 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.
  */
 #include <linux/slab.h>
 #include <linux/mutex.h>