hwmon: (max6697) Fix build warning

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

drivers/hwmon/max6697.c:653:34: warning:
	‘max6697_of_match’ defined but not used

Mark max6697_of_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/max6697.c

@@ -650,7 +650,7 @@ static const struct i2c_device_id max6697_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, max6697_id);
 
-static const struct of_device_id max6697_of_match[] = {
+static const struct of_device_id __maybe_unused max6697_of_match[] = {
 	{
 		.compatible = "maxim,max6581",
 		.data = (void *)max6581