scsi: mpt3sas: Add an I/O barrier

A barrier should be added to ensure proper ordering of memory mapped
writes.

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