firewire: core: shrink critical section of fw_card spinlock in bm_work

Now fw_core_handle_bus_reset() and bm_work() are serialized. Some members
of fw_card are free to access in bm_work()

This commit shrinks critical section of fw_card spinlock in bm_work()

Link: https://lore.kernel.org/r/20250917000347.52369-4-o-takashi@sakamocchi.jp


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