[SCSI] 3w-9xxx: scsi_dma_unmap fix

This patch fixes the following regression the occurred during the
scsi_dma_map()/unmap() changes:

3w-9xxx 0001:45:00.0: DMA-API: device driver tries to free DMA memory
it has not allocated [device address=0x0000000000000000] [size=36
bytes]
Signed-off-by: NAdam Radford <aradford@gmail.com>
Signed-off-by: NJames Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/3w-9xxx.c

@@ -1978,7 +1978,8 @@ static void twa_unmap_scsi_data(TW_Device_Extension *tw_dev, int request_id)
 {
 	struct scsi_cmnd *cmd = tw_dev->srb[request_id];
 
-	scsi_dma_unmap(cmd);
+	if (cmd->SCp.phase == TW_PHASE_SGLIST)
+		scsi_dma_unmap(cmd);
 } /* End twa_unmap_scsi_data() */
 
 /* scsi_host_template initializer */