firewire: core: use guard macro to disable local IRQ

The core function provides an operation for userspace application to
retrieve current value of CYCLE_TIMER register with several types of
system time. In the operation, local interrupt is disables so that the
access of the register and ktime are done atomically.

This commit uses guard macro to disable/enable local interrupts.

Link: https://lore.kernel.org/r/20240805085408.251763-9-o-takashi@sakamocchi.jp


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