ASoC: wm8804: Drop duplicate const

SOC_ENUM_SINGLE_DECL() already includes a const, drop the extra const.
Fixes the following sparse warning:

	sound/soc/codecs/wm8804.c:101:14: warning: duplicate const

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>