firewire: ohci: sanity-check MMIO resource

pci_request_region() does not fail on resources that have not been
allocated by the BIOS or by the kernel, so to avoid accessing
registers that are not there, we have to check for this explicitly.

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