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>