hwmon: (xdpe12284) Fix build warning seen if CONFIG_SENSORS_XDPE122_REGULATOR is disabled

0-day reports:

drivers/hwmon/pmbus/xdpe12284.c:127:36: warning:
	unused variable 'xdpe122_reg_desc'

This is seen if CONFIG_SENSORS_XDPE122_REGULATOR is not enabled.
Mark xdpe122_reg_desc as __maybe_unused to fix the problem.

Fixes: f53bfe4d ("hwmon: (xdpe12284) Add regulator support")
Reported-by: kernel test robot <lkp@intel.com>
Cc: Marcello Sylvester Bauer <sylv@sylv.io>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>