firewire: core: schedule bm_work item outside of spin lock

Before (re)building topology tree, fw_core_handle_bus_reset() schedules
a work item under acquiring fw_card spin lock. The work item invokes
bm_work() which acquires the spin lock at first, then can be stalled to
wait until the building tree finishes. This is inconvenient.

This commit moves the timing to schedule the work item after releasing
the spin lock.

Link: https://lore.kernel.org/r/20250917000347.52369-2-o-takashi@sakamocchi.jp


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