ASoC: rt5660: remove double const

commit 4281fcc02ed9f902dfa52d3635ac7f04b1a7341f upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>