hwmon: (pmbus) Fix krealloc() misuse in pmbus_add_attribute()

If krealloc() returns NULL, it *doesn't* free the original. So any code
of the form 'foo = krealloc(foo, …);' is almost certainly a bug.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>