firewire: core: minor code refactoring to localize table of gap count

The table for gap count is accessed by a single function. In this case,
it can be localized to the function.

Link: https://lore.kernel.org/r/20250617004320.477421-1-o-takashi@sakamocchi.jp


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