- Add checks for the (very unlikely) cases that the target writes too
   little or too much status data or writes unsolicited status.
 - Indicate that these and similar conditions are unlikely().
 - Check the 'resp' and 'sbp_status' fields for possible failure status.
 - Slightly optimize access macros for the status block bitfields.
 - Unify a few related log messages.

TODO:  Check if 'src'==1, then withhold the respective ORB from reuse
until status for any subsequent ORB was received.  This is an old bug
whose fix requires more complex command queue handling.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>