ieee1394: sbp2: stricter dma_sync

Two dma_sync_single_for_cpu() were called in the wrong place.
Luckily they were merely for DMA_TO_DEVICE, hence nobody noticed.

Also reorder the matching dma_sync_single_for_device() a little bit
so that they reside in the same functions as their counterparts.
This also avoids syncing the s/g table for requests which don't use it.

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