firewire: core: correct documentation of fw_csr_string() kernel API

Against its current description, the kernel API can accepts all types of
directory entries.

This commit corrects the documentation.

Cc: stable@vger.kernel.org
Fixes: 3c2c58cb ("firewire: core: fw_csr_string addendum")
Link: https://lore.kernel.org/r/20240130100409.30128-2-o-takashi@sakamocchi.jp


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