firewire: fw-sbp2: fix for SYM13FW500 bridge (Datafab disk)

Fix I/O errors due to SYM13FW500's inability to handle larger request
sizes.  Reported by Piergiorgio Sartor <piergiorgio.sartor@nexgo.de> in
https://bugzilla.redhat.com/show_bug.cgi?id=436879

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jarod Wilson <jwilson@redhat.com>