firewire: ohci: use guard macro to maintain bus time

The 1394 OHCI driver maintains bus time to respond to querying request.
The concurrent access to the bus time is protected by spinlock.

This commit uses guard macro to maintain the spinlock.

Link: https://lore.kernel.org/r/20240805085408.251763-16-o-takashi@sakamocchi.jp


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