ASoC: ak5558: mark OF related data as maybe unused

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

  sound/soc/codecs/ak5558.c:418:34: warning: ‘ak5558_i2c_dt_ids’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-2-krzk@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/ak5558.c

@@ -415,7 +415,7 @@ static int ak5558_i2c_remove(struct i2c_client *i2c)
 	return 0;
 }
 
-static const struct of_device_id ak5558_i2c_dt_ids[] = {
+static const struct of_device_id ak5558_i2c_dt_ids[] __maybe_unused = {
 	{ .compatible = "asahi-kasei,ak5558"},
 	{ }
 };