ASoC: ak4458: Modify SPDX license format

Modify SPDX comment style to match the rules. Since the original
code had a GPL-2.0 license and SPDX is compatible with GPL-2.0+,
we also modified the MODULE_LICENSE.
Signed-off-by: Cosmin-Gabriel Samoila <cosmin.samoila@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/ak4458.c

 // SPDX-License-Identifier: GPL-2.0
-/*
- * Audio driver for AK4458 DAC
- *
- * Copyright (C) 2016 Asahi Kasei Microdevices Corporation
- * Copyright 2018 NXP
- */
+//
+// Audio driver for AK4458 DAC
+//
+// Copyright (C) 2016 Asahi Kasei Microdevices Corporation
+// Copyright 2018 NXP
 
 #include <linux/module.h>
 #include <linux/delay.h>
@@ -655,4 +654,4 @@ module_i2c_driver(ak4458_i2c_driver);
 MODULE_AUTHOR("Junichi Wakasugi <wakasugi.jb@om.asahi-kasei.co.jp>");
 MODULE_AUTHOR("Mihai Serban <mihai.serban@nxp.com>");
 MODULE_DESCRIPTION("ASoC AK4458 DAC driver");
-MODULE_LICENSE("GPL");
+MODULE_LICENSE("GPL v2");

sound/soc/codecs/ak4458.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Audio driver for AK4458
  *