firewire: ohci: replace hard-coded values with inline functions for asynchronous packet header

This commit replaces the hard-coded values with the common inline functions
to serialize and deserialize the header of asynchronous packet.

Link: https://lore.kernel.org/r/20240428071347.409202-5-o-takashi@sakamocchi.jp


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