sg_dma_len(sg) is invalid before the s/g list is DMA-mapped.

This fixes a post 2.6.24 regression which prevents access to SBP-2
devices on several architectures, introduced by "ieee1394: sbp2: s/g
list access cosmetics", commit 825f1df5.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>