ieee1394: sbp2: better handling of transport errors

If the target signals a transport failure via status block, complete the
request with DID_BUSY to indicate to the SCSI subsystem that the command
may succeed when retried.

Also log diagnostic information if the status block shows a transport
related problem.  It may point to hardware faults.

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