firewire: core: minor code refactoring for topology builder

Current implementation to build tree according to self ID sequences has
the rest to be refactored; e.g. putting local variables into block.

This commit is for the purpose.

Link: https://lore.kernel.org/r/20240605235155.116468-4-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>