driver core: Add a comment to set_primary_fwnode() on nullifying

Explain what parent && fn == parent->fwnode conditional does.
With this refactor the code a bit.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230323182640.61085-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>