hwmon: (pmbus/ucd900) Fix build warning

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

drivers/hwmon/pmbus/ucd9000.c:154:34: warning:
	‘ucd9000_of_match’ defined but not used

Mark ucd9000_of_match as __maybe_unused to fix the problem.

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