hwmon: (ina209) Fix build warning

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

drivers/hwmon/ina209.c:590:34: warning:
	‘ina209_of_match’ defined but not used

Mark ina209_of_match as __maybe_unused to fix the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>