firewire: core: add tests for serialization/deserialization of phy config packet

firewire: core: add tests for serialization/deserialization of phy config packet

In the protocol of IEEE 1394, phy configuration packet is broadcasted to
the bus to configure all PHYs residing on the bus. It includes two
purposes; selecting root node and optimizing gap count.

This commit adds some helper function to serialize/deserialize the
content of phy configuration packet, as well as some KUnit tests for it.

Link: https://lore.kernel.org/r/20240606235133.231543-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>