scsi-disk: fix the check of the DBD bit in the MODE SENSE command

The DBD bit does not work as expected.

SCSI-Spec:
http://ldkelley.com/SCSI2/SCSI2/SCSI2-08.html#8.2.10
"A disable block descriptors (DBD) bit of zero indicates that the target
may return zero or more block descriptors in the returned MODE SENSE
data (see 8.3.3), at the target's discretion. A DBD bit of one
specifies that the target shall not return any block descriptors in the
returned MODE SENSE data."

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
