firewire: core: use guard macro to maintain list of events for userspace clients

The core function maintains events to userspace by list in the instance of
client. The concurrent access to the list is protected by spinlock in
the instance.

This commit uses guard macro to maintain the spinlock.

Link: https://lore.kernel.org/r/20240805085408.251763-10-o-takashi@sakamocchi.jp


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