firewire: cdev: fix documentation of FW_CDEV_IOC_GET_INFO

The FW_CDEV_IOC_GET_INFO ioctl looks at client->device->config_rom, not
at the local node's config ROM.

We could fix the implementation or the documentation.  I believe the way
how it is currently implemented is more useful than the way how it is
currently documented.  In fact, libdc1394 uses the ABI already as
implemented, not as documented.  Hence let's change the documentation.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>