firewire: core: use guard macro to maintain isochronous context for userspace client

The core function allows one isochronous contexts per userspace client.
The concurrent access to the context is protected by spinlock in the
instance of client.

This commit uses guard macro to maintain the spinlock.

Link: https://lore.kernel.org/r/20240805085408.251763-12-o-takashi@sakamocchi.jp


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