sbp2: proper treatment of DID_OK

Sbp2 relied on DID_OK to be defined as 0. Always shift DID_OK into the right
position anyway, and explicitly return DID_OK together with CHECK_CONDITION.
Also comment on some #if 0 code. The patch does not change current behaviour.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>