mpt3sas: Build MPI SGL LIST on GEN2 HBAs and IEEE SGL LIST on GEN3 HBAs

Gen2 HBAs use MPI scatter-gather lists whereas Gen3 HBAs use IEEE
scatter-gather lists. Modify the common code part in such a way that it
will build IEEE SGL tables for Gen3 HBAs and MPI SGL tables for Gen2
HBAs.

Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>