firewire: fix struct fw_node memory leak

With the bus_resets patch applied, it is easy to see this memory leak
by repeatedly resetting the firewire bus while running slabtop in
another window.  Just watch kmalloc-32 grow and grow...

Signed-off-by: Jay Fenlason <fenlason@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>