firewire: core: add is_local sysfs device attribute

Making this information available in sysfs allows to differentiate
between controllers in the local and remote Linux PCs, and thus is
useful for servers that are started with udev rules.

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