mpt3sas: Set maximum transfer length per IO to 4MB for VDs

Set maximum transfer length per IO on RAID volumes to 4MB by setting
VD's queue's max_sector to 8192.

Signed-off-by: Chaitra P B <chaitra.basappa@broadcom.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>