firewire: Use device->groups for adding device attributes.

We dynamically create an attribute group for the key present on the
device in hand and point device->group to it.  This way the device
core adds the sysfs attributes for us as the device is added.

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