firewire: core: add helper function to handle port status from self ID sequence and its KUnit test

The self ID sequence delivers the information about the state of port.

This commit adds some enumerations to express the state of port, and
some helper functions to handle the state. It adds a KUnit test for them,
too.

Link: https://lore.kernel.org/r/20240605235155.116468-3-o-takashi@sakamocchi.jp


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