scsi: megaraid_sas: optimize raid context access in IO path

No functional change.  Use local variables when accessing raid context in
IO path.  Improves code readability.

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>