test_printf: Refactor fwnode_pointer() to make it more readable

Converting fwnode_pointer() to use better swnode API allows to
make code more readable.

While at it, rename full_name to full_name_third to show exact
relation in the hierarchy.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220824170542.18263-1-andriy.shevchenko@linux.intel.com