scsi: arcmsr: Send SYNCHRONIZE_CACHE command to firmware

The arcmsr driver failed to pass SYNCHRONIZE CACHE to controller
firmware. Depending on how drive caches are handled internally by
controller firmware this could potentially lead to data integrity
problems.

Ensure that cache flushes are passed to the controller.

[mkp: applied by hand and removed unused vars]

Cc: <stable@vger.kernel.org>
Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Reported-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>