firewire: core: fix crash in iso resource management

This fixes a regression due to post 2.6.30 commit "firewire: core: do
not DMA-map stack addresses" 6fdc0370

.

As David Moore noted, a previously correct sizeof() expression became
wrong since the commit changed its argument from an array to a pointer.
This resulted in an oops in ohci_cancel_packet in the shared workqueue
thread's context when an isochronous resource was to be freed.

Reported-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>