firewire: core: use scoped_guard() to manage critical section to update topology

At present, guard() macro is used for the critical section to update
topology. It is inconvenient to add the other critical sections into
the function.

This commit uses scoped_guard() macro instead.

Link: https://lore.kernel.org/r/20250915234747.915922-2-o-takashi@sakamocchi.jp


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