asoc: Update file header to GPL-2.0-only

Update file header to GPL-2.0-only.

Change-Id: If16bf92f951c6d7521f56a3e9b91da2ca2ccf6f8
Signed-off-by: Meng Wang <mengw@codeaurora.org>

dsp/q6usm.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright (c) 2011-2014, 2018, The Linux Foundation. All rights reserved.
  */