firewire: ohci: use guard macro to serialize accesses to phy registers

The 1394 OHCI driver protects concurrent accesses to phy registers by
mutex object in fw_ohci structure.

This commit uses guard macro to maintain the mutex.

Link: https://lore.kernel.org/r/20240805085408.251763-5-o-takashi@sakamocchi.jp


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