hwmon: (adc128d818) Fix build warning

If CONFIG_OF is not enabled, the following build warning is observed.

drivers/hwmon/adc128d818.c:524:34: warning:
	‘adc128_of_match’ defined but not used

Mark adc128_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/adc128d818.c

@@ -521,7 +521,7 @@ static const struct i2c_device_id adc128_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, adc128_id);
 
-static const struct of_device_id adc128_of_match[] = {
+static const struct of_device_id __maybe_unused adc128_of_match[] = {
 	{ .compatible = "ti,adc128d818" },
 	{ },
 };