[SCSI] libsas: Unknown STP devices should be reported to libata as unknown.

When libsas encounters a STP device whose protocol isn't recognized (i.e.
not ATA or ATAPI), we should set the ata_device's class to ATA_DEV_UNKNOWN
instead of ATA_DEV_ATA.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>