ieee1394: dv1394: fix CardBus card ejection

Fix NULL pointer dereference on hot ejection of a FireWire card while
dv1394 was loaded.  http://bugzilla.kernel.org/show_bug.cgi?id=7121
I did not test card ejection with open /dev/dv1394 files yet.

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