firewire: core: Open-code topology list walk

firewire: core: Open-code topology list walk

A later change will make list_for_each_entry() cache the next element
before entering the loop body. for_each_fw_node() intentionally appends
newly discovered child nodes to the temporary walk list while the list is
being traversed.

Keep the loop open-coded so the next node is looked up only after
children have been appended. This preserves the current breadth-first
traversal semantics and prepares the code for the list iterator update.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Link: https://lore.kernel.org/r/20260609061347.93688-3-kaitao.cheng@linux.dev
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>