scsi: arcmsr: Send SYNCHRONIZE_CACHE command to firmware

commit 2bf7dc8443e113844d078fd6541b7f4aa544f92f upstream.

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]

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>
Signed-off-by: Willy Tarreau <w@1wt.eu>