scsi: aacraid: Preserve MSIX mode in the OMR register

Preserve the current MSIX mode value in the OMR before rewriting the OMR
to initiate the IOP or Soft Reset.

Signed-off-by: Prasad B Munirathnam <prasad.munirathnam@microsemi.com>
Signed-off-by: Raghava Aditya Renukunta <RaghavaAditya.Renukunta@microsemi.com>
Reviewed-by: Dave Carroll <david.carroll@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>