hwmon: (ads7828) Fix build warning

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

drivers/hwmon/ads7828.c:203:34: warning:
	‘ads7828_of_match’ defined but not used

Mark ads7828_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ads7828.c

@@ -200,7 +200,7 @@ static const struct i2c_device_id ads7828_device_ids[] = {
 };
 MODULE_DEVICE_TABLE(i2c, ads7828_device_ids);
 
-static const struct of_device_id ads7828_of_match[] = {
+static const struct of_device_id __maybe_unused ads7828_of_match[] = {
 	{
 		.compatible = "ti,ads7828",
 		.data = (void *)ads7828