target: Fix zero-length INQUIRY additional sense code regression

This patch fixes a minor regression introduced in v3.8-rc1 code
where a zero-length INQUIRY was no longer returning the correct
INVALID FIELD IN CDB additional sense code.

This regression was introduced with the following commit:

  commit de103c93
  Author: Christoph Hellwig <hch@lst.de>
  Date:   Tue Nov 6 12:24:09 2012 -0800

      target: pass sense_reason as a return value

and this patch has been tested with the following zero-length CDB:

  sg_raw /dev/sdd 12 00 83 00 00 00
  SCSI Status: Check Condition

  Sense Information:
   Fixed format, current;  Sense key: Illegal Request
   Additional sense: Invalid field in cdb

Cc: Christoph Hellwig <hch@lst.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>