scsi: mpt3sas: Recognize and act on iopriority info

This patch adds support for request iopriority handling in the mpt3sas
layer. This works only when a ATA device is behind the SATL. The ATA
device also has to indicate that it supports command priorities in the
identify information that is pulled from the SATL.

Signed-off-by: Adam Manzanares <adam.manzanares@wdc.com>
Acked-by: Sreekanth Reddy <Sreekanth.Reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>