ieee1394: fix NULL pointer dereference in sysfs access

Regression since "ieee1394: prevent device binding of raw1394,
video1394, dv1394", commit d2ace29f:
$ cat /sys/bus/ieee1394/drivers/raw1394/device_ids
triggers a NULL pointer dereference in fw_show_drv_device_ids.
Reported by Miles Lane.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Miles Lane <miles.lane@gmail.com>