firewire: core: refer fw_card member to initiate bus reset under acquiring lock

The gap_count member of fw_card structure is referred when initiate bus
reset. This reference is done out of acquiring lock. This is not good.

This commit takes the reference within the acquiring lock, with
additional code refactoring.

Link: https://lore.kernel.org/r/20250908012108.514698-9-o-takashi@sakamocchi.jp


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