firewire: cdev: code refactoring to dispatch event for phy packet

In 1394 OHCI, both Asynchronous Transmit (AT) and Asynchronous Receive
(AR) contexts are used to deliver the phy packet of IEEE 1394. The time
stamp is available as well as the usual asynchronous transaction.

This commit is a preparation for future commit to handle the time stamp.

Link: https://lore.kernel.org/r/20230529113406.986289-11-o-takashi@sakamocchi.jp


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