scsi: consolidate opcode lookup in scsi_opcode_sa_name()

Consolidate the CDB opcode lookup in scsi_opcode_sa_name(),
so that we don't have to call several functions to figure
out the CDB opcode string.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>