ASoC: codecs: adau1977-spi: Mark OF related data as maybe unused

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

  sound/soc/codecs/adau1977-spi.c:58:34: error: ‘adau1977_spi_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/20230310214333.274903-6-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/adau1977-spi.c

@@ -55,7 +55,7 @@ static const struct spi_device_id adau1977_spi_ids[] = {
 };
 MODULE_DEVICE_TABLE(spi, adau1977_spi_ids);
 
-static const struct of_device_id adau1977_spi_of_match[] = {
+static const struct of_device_id adau1977_spi_of_match[] __maybe_unused = {
         { .compatible = "adi,adau1977" },
         { .compatible = "adi,adau1978" },
         { .compatible = "adi,adau1979" },