firewire: ohci: fix DMA unmapping in an error path

If request_irq failed, we would pass wrong arguments to
dma_free_coherent.  https://bugzilla.redhat.com/show_bug.cgi?id=728185

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