firewire: sbp2: fix payload limit at S1600 and S3200

1394-2008 clause 16.3.4.1 (1394b-2002 clause 16.3.1.1) defines tighter
limits than 1394-2008 clause 6.2.2.3 (1394a-2000 clause 6.2.2.3).

Our previously too large limit doesn't matter though if the controller
reports its max_receive correctly.

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