scsi: mpt3sas: remove redundant wmb

Due to relaxed ordering requirements on multiple architectures, drivers
are required to use wmb/rmb/mb combinations when they need to guarantee
observability between the memory and the HW.

The mpt3sas driver is already using wmb() for this purpose.  However, it
issues a writel following wmb(). writel() function on arm/arm64
arhictectures have an embedded wmb() call inside.

This results in unnecessary performance loss and code duplication.

writel already guarantees ordering for both cpu and bus. we don't need
additional wmb()

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Acked-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>