firewire: core: use macro expression for gap count mismatch

The gap_count field is assigned to zero when mismatch is detected. In such
case, the macro expression is preferable since it is easy to understand
the situation.

This commit applies the idea.

Link: https://lore.kernel.org/r/20250913105737.778038-2-o-takashi@sakamocchi.jp


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