scsi: mpt3sas: change _base_get_msix_index prototype

Code refactoring.

In function _base_get_msix_index, add scmd as second argument. This change
is made in preparation for the next patch where we introduce a new function
to get the MSI-X index for high iops queues.

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