scsi: mpt3sas: save and use MSI-X index for posting RD

In the IO submission path _base_get_msix_index is called twice. Initially
while getting the smid and subsequently while posting the request
descriptor (RD).

Refactor code to query msix index only while posting the request
descriptor. Save determined msix index in msix_io field.

Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>