[SCSI] scsi_transport_sas: add 12GB definitions for mpt3sas

[jejb: split this core change into a separate patch]
Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@lsi.com>
Reviewed-by: Nagalakshmi Nandigama <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>