firewire: ohci: use guard macro to serialize operations for isochronous contexts

The 1394 OHCI driver uses spinlock to serialize operations for
isochronous contexts.

This commit uses guard macro to maintain the spinlock.

Link: https://lore.kernel.org/r/20240805085408.251763-18-o-takashi@sakamocchi.jp


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