hwmon: (pmbus) Initialize sysfs attributes

Initialize sysfs attrs before device_create_file() call to suppress
lockdep_init_map() warning:

WARNING: at kernel/lockdep.c:2701 lockdep_init_map+0x12d/0x140()

Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: stable@kernel.org # .39.x