firewire: core: adds constant qualifier for local helper functions

Some local functions just handles given argument as mutable, thus it is
preferable to add constant qualifier to them.

Link: https://lore.kernel.org/r/20231221134849.603857-2-o-takashi@sakamocchi.jp


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