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

sound/soc/samsung/smdk_wm8994.c

@@ -136,7 +136,7 @@ static struct snd_soc_card smdk = {
 	.num_links = ARRAY_SIZE(smdk_dai),
 };
 
-static const struct of_device_id samsung_wm8994_of_match[] = {
+static const struct of_device_id samsung_wm8994_of_match[] __maybe_unused = {
 	{ .compatible = "samsung,smdk-wm8994", .data = &smdk_board_data },
 	{},
 };