ASoC: samsung: smdk_wm8994: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  sound/soc/samsung/smdk_wm8994.c:139:34: warning: ‘samsung_wm8994_of_match’ defined but not used [-Wunused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-11-krzk@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>