ieee1394: fix oops on "modprobe -r ohci1394" after network class_device conversion

The networking subsystem has been converted from class_device to device
but ieee1394 hasn't.  This results in a 100% reproducible NULL pointer
dereference if the ohci1394 driver module is unloaded while the eth1394
module is still loaded.
http://lkml.org/lkml/2006/11/16/147
http://lkml.org/lkml/2007/3/14/4

This is a regression in 2.6.21-rc1.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Ismail Dönmez <ismail@pardus.org.tr>