ASoC: pcm179x: skip of_device_id table when !CONFIG_OF

The driver can match by multiple methods.  Its of_device_id table is
referenced via of_match_ptr() so it will be unused for !CONFIG_OF
builds:

  sound/soc/codecs/pcm179x-i2c.c:33:34: warning: ‘pcm179x_of_match’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201125164452.89239-29-krzk@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/pcm179x-i2c.c

@@ -30,11 +30,13 @@ static int pcm179x_i2c_probe(struct i2c_client *client,
 	return pcm179x_common_init(&client->dev, regmap);
 }
 
+#ifdef CONFIG_OF
 static const struct of_device_id pcm179x_of_match[] = {
 	{ .compatible = "ti,pcm1792a", },
 	{ }
 };
 MODULE_DEVICE_TABLE(of, pcm179x_of_match);
+#endif
 
 static const struct i2c_device_id pcm179x_i2c_ids[] = {
 	{ "pcm179x", 0 },