In both core and 1394 OHCI driver, some hard-coded values and macros are
used to serialize/deserialize the header of asynchronous packets. It is
inconvenient to reuse them.

This commit adds some helper inline functions with their tests for the
purpose.

Link: https://lore.kernel.org/r/20240428071347.409202-2-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>