firewire: Make use of struct device_type.

The device_type struct is useful for setting attributes for a device
and overriding .release and .uevent for a group of devices.  This patch
uses it this way to clean up the sysfs code a bit.

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