hwmon: (max6650) Fix build warning

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

drivers/hwmon/max6650.c:137:34: warning:
	‘max6650_dt_match’ defined but not used

Mark max6650_dt_match as __maybe_unused to fix the problem.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/max6650.c

@@ -134,7 +134,7 @@ static const u8 tach_reg[] = {
 	MAX6650_REG_TACH3,
 };
 
-static const struct of_device_id max6650_dt_match[] = {
+static const struct of_device_id __maybe_unused max6650_dt_match[] = {
 	{
 		.compatible = "maxim,max6650",
 		.data = (void *)1