[SCSI] sd: fix USB devices incorrectly reporting DIF support

Some USB devices set the protect bit in the INQUIRY data which
currently causes the DIF code in sd to assume (incorrectly) that they
support READ_CAPACITY(16).  Fix this (only for the time being) by
making sure we only believe the protect bit in the inquiry data if the
device claims conformance to SCSI-3 or above.

Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>