hwmon: (max6621) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/max6621.c:573:34: warning:
	‘max6621_of_match’ defined but not used

Mark max6621_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/max6621.c

@@ -552,7 +552,7 @@ static const struct i2c_device_id max6621_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, max6621_id);
 
-static const struct of_device_id max6621_of_match[] = {
+static const struct of_device_id __maybe_unused max6621_of_match[] = {
 	{ .compatible = "maxim,max6621" },
 	{ }
 };