firewire: core: use spin lock specific to timer for split transaction

At present the parameters to compute timeout time for split transaction is
protected by card-wide spin lock, while it is not necessarily convenient
in a point to narrower critical section.

This commit adds and uses another spin lock specific for the purpose.

Link: https://lore.kernel.org/r/20250915234747.915922-6-o-takashi@sakamocchi.jp


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