ASoC: ma120x0p: Remove unnecessary const specifier

Clang warns:

  sound/soc/codecs/ma120x0p.c:891:14: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
  static const SOC_VALUE_ENUM_SINGLE_DECL(pwr_mode_ctrl,
               ^
  ./include/sound/soc.h:362:2: note: expanded from macro 'SOC_VALUE_ENUM_SINGLE_DECL'
          SOC_VALUE_ENUM_DOUBLE_DECL(name, xreg, xshift, xshift, xmask, xtexts, xvalues)
          ^
  ./include/sound/soc.h:359:2: note: expanded from macro 'SOC_VALUE_ENUM_DOUBLE_DECL'
          const struct soc_enum name = SOC_VALUE_ENUM_DOUBLE(xreg, xshift_l, xshift_r, xmask, \
          ^
  1 warning generated.

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

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