firewire: cdev: code refactoring to operate event of response

This commit is a preparation to handle time stamp of asynchronous
transaction for user space application.

Link: https://lore.kernel.org/r/20230529113406.986289-8-o-takashi@sakamocchi.jp


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