ASoC: rt5659: drop 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() as well as the
double const by calls to SOC_VALUE_ENUM_SINGLE_DECL() via
SOC_VALUE_ENUM_DOUBLE_DECL).

Fixes: commit d3cb2de2 ("ASoC: rt5659: add rt5659 codec driver")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>