Within FireWire subsystem, the serializations and deserializations of phy
packet are implemented in several parts. They includes some redundancies.

This commit adds a series of helper functions for the serializations and
deserializations of self ID packet with a Kunit test suite.

Link: https://lore.kernel.org/r/20240605235155.116468-8-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>