scsi: mpt3sas: Introduce mpt3sas_get_st_from_smid()

Abstract accesses to the scsi_lookup array by introducing
mpt3sas_get_st_from_smid().

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