firewire: Switch cdev code over to use register_chrdev and keep a list of devices.

The old mechanism kept a struct cdev for each fw device, but fops->release
would reference this struct after the device got freed in some cases.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>