firewire: core: fix build failure due to the caller of fw_csr_string()

A commit 47dc5518 ("firewire: core: search descriptor leaf just after
vendor directory entry in root directory") for v6.8-rc3 and a commit
67a5a58c0443 ("firewire: Kill unnecessary buf check in
device_attribute.show") for v6.9 bring build failure in for-next tree due
to the change of the name of local variable.

This commit fixes it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20240202111602.6f6e2c1a@canb.auug.org.au/


Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402022343.NkgsMITA-lkp@intel.com/
Link: https://lore.kernel.org/r/20240205060448.13881-1-o-takashi@sakamocchi.jp


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