hwmon: (pmbus/tps53679) Fix build warning

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

drivers/hwmon/pmbus/tps53679.c:100:34: warning:
	‘tps53679_of_match’ defined but not used

Marking tps53679_of_match as __maybe_unused fixes the problem.

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