ASoC: rt5514: fix gcc-7 warning

gcc-7 warns that there is a duplicate 'const' specifier in some
variables that are declared using the SOC_ENUM_SINGLE_DECL macro:

sound/soc/codecs/rt5514.c:398:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(
sound/soc/codecs/rt5514.c:405:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(

This removes one to fix the warning.

Fixes: 4a6180ea

 ("ASoC: rt5514: add rt5514 codec driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>