ASoC: tlv320aic31xx: Remove duplicate const

SOC_ENUM_SINGLE_DECL() already adds the const qualifier, so there is no need to
manually specify it.

Fixes the following warnings from sparse:
	sound/soc/codecs/tlv320aic31xx.c:253:1: warning: duplicate const
	sound/soc/codecs/tlv320aic31xx.c:255:1: warning: duplicate const
	sound/soc/codecs/tlv320aic31xx.c:257:1: warning: duplicate const
	sound/soc/codecs/tlv320aic31xx.c:260:1: warning: duplicate const
	sound/soc/codecs/tlv320aic31xx.c:262:1: warning: duplicate const

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>