firewire: net: fix NULL derefencing in fwnet_probe()

"dev" and "net" are NULL when alloc_netdev() is failed.
So just unlock and return an error.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>