firewire: Kill unnecessary buf check in device_attribute.show

Per Documentation/filesystems/sysfs.rst:
> sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
> method.

So we can kill the unnecessary buf check safely.

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Link: https://lore.kernel.org/r/20240122053942.80648-1-lizhijian@fujitsu.com


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