ASoC: rt5665: 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/rt5665.c:915:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(rt5665_if1_1_01_adc_enum,
sound/soc/codecs/rt5665.c:918:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(rt5665_if1_1_23_adc_enum,
sound/soc/codecs/rt5665.c:921:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(rt5665_if1_1_45_adc_enum,
sound/soc/codecs/rt5665.c:924:14: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
 static const SOC_ENUM_SINGLE_DECL(rt5665_if1_1_67_adc_enum,
...

This removes one to fix the 68 warnings in this file

Fixes: 33ada14a ("ASoC: add rt5665 codec driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>