ASoC: ak4458: make structure soc_codec_dev_ak4458 static const

The structure soc_codec_dev_ak4458 is local to the source and do not
need to be in global scope and can be const, make it static const.

Cleans up sparse warnings:
warning: symbol 'soc_codec_dev_ak4458' was not declared. Should it
be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/ak4458.c

@@ -558,7 +558,7 @@ static int __maybe_unused ak4458_runtime_resume(struct device *dev)
 }
 #endif /* CONFIG_PM */
 
-struct snd_soc_component_driver soc_codec_dev_ak4458 = {
+static const struct snd_soc_component_driver soc_codec_dev_ak4458 = {
 	.probe			= ak4458_probe,
 	.remove			= ak4458_remove,
 	.controls		= ak4458_snd_controls,