ASoC: codecs: pcm179x-spi: Mark OF related data as maybe unused

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

  sound/soc/codecs/pcm179x-spi.c:32:34: error: ‘pcm179x_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230310214333.274903-7-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/pcm179x-spi.c

@@ -29,7 +29,7 @@ static int pcm179x_spi_probe(struct spi_device *spi)
 	return pcm179x_common_init(&spi->dev, regmap);
 }
 
-static const struct of_device_id pcm179x_of_match[] = {
+static const struct of_device_id pcm179x_of_match[] __maybe_unused = {
 	{ .compatible = "ti,pcm1792a", },
 	{ }
 };