firewire: core: use helper macros instead of direct access to HZ

There are some macros available to convert usecs, msecs, and secs into
jiffies count.

Link: https://lore.kernel.org/r/20250915024232.851955-2-o-takashi@sakamocchi.jp


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