firewire: core: code refactoring to evaluate transaction result to CSR_BUS_MANAGER_ID

The call of bm_work should be done after acquiring spin lock of fw_card.
For asynchronous transaction, the lock should be released temporarily
due to event waiting.

A commit 27310d56 ("firewire: core: use guard macro to maintain
properties of fw_card") applied scoped_guard() to the bm_work function,
however it looks hard to follow to the control flow.

This commit refactors the spin lock acquisition after the transaction.

Link: https://lore.kernel.org/r/20250908012108.514698-8-o-takashi@sakamocchi.jp


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