ASoC: ak5558: make two structures static

The structure ak5558_pm and soc_codec_dev_ak5558 are local to the
source and do not need to be in global scope, so make them static.
Also make soc_codec_dev_ak5558 static.

Cleans up sparse warnings:
warning: symbol 'ak5558_pm' was not declared. Should it be static?
warning: symbol 'soc_codec_dev_ak5558' was not declared. Should it be
static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>