ASoC: bcm: allo-piano-dac-plus: Remove unnecessary const specifiers

Clang warns:

  sound/soc/bcm/allo-piano-dac-plus.c:66:14: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
  static const SOC_ENUM_SINGLE_DECL(allo_piano_mode_enum,
               ^
  ./include/sound/soc.h:355:2: note: expanded from macro 'SOC_ENUM_SINGLE_DECL'
          SOC_ENUM_DOUBLE_DECL(name, xreg, xshift, xshift, xtexts)
          ^
  ./include/sound/soc.h:352:2: note: expanded from macro 'SOC_ENUM_DOUBLE_DECL'
          const struct soc_enum name = SOC_ENUM_DOUBLE(xreg, xshift_l, xshift_r, \
          ^
  sound/soc/bcm/allo-piano-dac-plus.c:75:14: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
  static const SOC_ENUM_SINGLE_DECL(allo_piano_dual_mode_enum,
               ^
  ./include/sound/soc.h:355:2: note: expanded from macro 'SOC_ENUM_SINGLE_DECL'
          SOC_ENUM_DOUBLE_DECL(name, xreg, xshift, xshift, xtexts)
          ^
  ./include/sound/soc.h:352:2: note: expanded from macro 'SOC_ENUM_DOUBLE_DECL'
          const struct soc_enum name = SOC_ENUM_DOUBLE(xreg, xshift_l, xshift_r, \
          ^
  sound/soc/bcm/allo-piano-dac-plus.c:96:14: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
  static const SOC_ENUM_SINGLE_DECL(allo_piano_enum,
               ^
  ./include/sound/soc.h:355:2: note: expanded from macro 'SOC_ENUM_SINGLE_DECL'
          SOC_ENUM_DOUBLE_DECL(name, xreg, xshift, xshift, xtexts)
          ^
  ./include/sound/soc.h:352:2: note: expanded from macro 'SOC_ENUM_DOUBLE_DECL'
          const struct soc_enum name = SOC_ENUM_DOUBLE(xreg, xshift_l, xshift_r, \
          ^
  3 warnings generated.

SOC_VALUE_ENUM_DOUBLE_DECL already has a const specifier. Remove the duplicate
const specifiers to clean up the warnings.

Fixes: 42444979 ("Add support for all the downstream rpi sound card drivers")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>