ASoC: cs42xx8: Make of match table static

The cs42xx8_of_match table is not used outside of the driver, hence it can and
should be made static.

Fixes the following warning from sparse:
	sound/soc/codecs/cs42xx8.c:425:27: warning: symbol 'cs42xx8_of_match' was
	not declared. Should it be static?

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@linaro.org>