hwmon: (ina2xx) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/ina2xx.c:510:34: warning:
	‘ina2xx_of_match’ defined but not used

Mark ina2xx_of_match as __maybe_unused to fix the problem.

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