lib/vsprintf: OF nodes are first and foremost, struct device_nodes

Factor out static kobject_string() function that simply calls
device_node_string(), and thus remove references to kobjects (as these are
struct device_node).

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>