firewire: core: add common macro to serialize/deserialize isochronous packet header

The packet for Asynchronous Streaming Packet includes the same header
fields as the isochronous packet has. It is helpful to have some helper
functions to serialize/deserialize them.

This commit adds such helper functions with their test.

Link: https://lore.kernel.org/r/20240428071347.409202-8-o-takashi@sakamocchi.jp


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