firewire: Use dma_mapping_error() for checking for DMA mapping errors.

Pointed out by Pete Zaitcev.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

diff --cc drivers/firewire/fw-iso.c
Simple merge

diff --cc drivers/firewire/fw-ohci.c
Simple merge

diff --cc drivers/firewire/fw-sbp2.c
Simple merge