firewire: core: use guard macro to maintain list of receivers for phy configuration packets

The core function maintains clients to receive phy configuration packets
by list in the instance of fw_card. The concurrent access to the list is
protected by spinlock in the instance.

This commit uses guard macro to maintain the list.

Link: https://lore.kernel.org/r/20240805085408.251763-13-o-takashi@sakamocchi.jp


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