firewire: ohci: remove module-local workqueue

Now module-local workqueue has been replaced by a threaded IRQ handler.

This commit removes the workqueue and the associated work item
accordingly.

Link: https://lore.kernel.org/r/20250823030954.268412-4-o-takashi@sakamocchi.jp


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