firewire: fw-sbp2: fix DMA mapping of S/G tables

  - The CPU must not touch the buffer after it was DMA-mapped.
  - The size argument of dma_unmap_single(...page_table...) was bogus.
  - Move a comment closer to the code to which it refers to.

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