hwmon: (pmbus/ucd900) Fix build warning

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

drivers/hwmon/pmbus/ucd9000.c:154:34: warning:
	‘ucd9000_of_match’ defined but not used

Mark ucd9000_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/ucd9000.c

@@ -151,7 +151,7 @@ static const struct i2c_device_id ucd9000_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, ucd9000_id);
 
-static const struct of_device_id ucd9000_of_match[] = {
+static const struct of_device_id __maybe_unused ucd9000_of_match[] = {
 	{
 		.compatible = "ti,ucd9000",
 		.data = (void *)ucd9000