ASoC: rt5660: remove double const

Drop the const qualifier as it is being added by SOC_ENUM_DOUBLE_DECL()
already which is called by SOC_ENUM_SINGLE_DECL() here.

Fixes: commit 2b26dd4c ("ASoC: rt5660: add rt5660 codec driver")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>