firewire: fix warnings to generate UAPI documentation

firewire: fix warnings to generate UAPI documentation

Any target to generate UAPI documentation reports warnings to missing
annotation for padding member in structures added recently.

This commit suppresses the warnings.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20230531135306.43613a59@canb.auug.org.au/
Fixes: 7c22d4a92bb2 ("firewire: cdev: add new event to notify request subaction with time stamp")
Fixes: fc2b52cf2e0e ("firewire: cdev: add new event to notify response subaction with time stamp")
Link: https://lore.kernel.org/r/20230601144937.121179-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>