firewire: core: use guard macro to maintain RCU scope for transaction address handler

The core function maintains address handlers by list. RCU is utilized
for efficient read access to any entries in the list.

This commit uses guard macro to maintain RCU locking and releasing.

Link: https://lore.kernel.org/r/20240805085408.251763-6-o-takashi@sakamocchi.jp


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