firewire: core: use guard macro to maintain IDR of isochronous resources for userspace clients

The core function provides UAPI to maintain isochronous resources allocated
by userspace clients across bus resets automatically. The resources are
maintained by IDR and the concurrent access to it is protected by spinlock
in the instance of client.

This commit uses guard macro to maintain the spinlock.

Link: https://lore.kernel.org/r/20240805085408.251763-11-o-takashi@sakamocchi.jp


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