firewire: net: fix card driver reloading

Fix some problems from "firewire: net: allow for unordered unit
discovery":
  - fwnet_remove was missing a list_del, causing fwnet_probe to crash if
    called after fwnet_remove, e.g. if firewire-ohci was unloaded and
    reloaded.
  - fwnet_probe should set its new_netdev flag only if it actually
    allocated a net_device.
  - Use dev_set_drvdata and dev_get_drvdata instead of deprecated direct
    access to device.driver_data.

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