firewire: sbp2: fix status reception

Per SBP-2 clause 5.3, a target shall store 8...32 bytes of status
information.  Trailing zeros after the first 8 bytes don't need to be
stored, they are implicit.  Fix the status write handler to clear all
unwritten status data.

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