ASoC: wm8988: Constify struct regmap_config and snd_soc_codec_driver

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const. Make struct
snd_soc_codec_driver const as well (snd_soc_register_codec() accepts
pointer to const).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>