ASoC: update headers for upstream compliance

Update uapi header reference path to include
path prefix "audio/"

Change-Id: Ic7b1d22910b07be3ffe8d9f02514ed89facea974
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>

asoc/codecs/wcd9xxx-slimslave.c

@@ -1,9 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-only
-/* Copyright (c) 2012-2018, The Linux Foundation. All rights reserved.
+/* Copyright (c) 2012-2018, 2020 The Linux Foundation. All rights reserved.
  */
 #include <linux/slab.h>
 #include <linux/mutex.h>
-#include <linux/mfd/wcd9xxx/wcd9xxx_registers.h>
+#include <audio/linux/mfd/wcd9xxx/wcd9xxx_registers.h>
 #include <asoc/wcd9xxx-slimslave.h>
 
 struct wcd9xxx_slim_sch {