scsi: megaraid_sas: change issue_dcmd to return void from int

With the changes to remove checks for a valid request descriptor,
issue_dcmd will now always return DCMD_SUCCESS. This patch changes
return type of issue_dcmd to void and change all callers appropriately.

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>