firewire: consistent usage of node_id

Definitions as per IEEE 1212 and IEEE 1394:

     Node ID: Concatenation of bus ID and local ID. 16 bits long.
      Bus ID: Identifies a particular bus within a group of buses
              interconnected by bus bridges.
    Local ID: Identifies a particular node on a bus.
      PHY ID: Local ID of IEEE 1394 nodes. 6 bits long.

Never ever use a variable called node_id for anything else than a node ID.

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