[SCSI] fix panic when ejecting ieee1394 ipod

The scsi_library routines don't correctly set DMA_NONE when
req->data_len is zero (instead they check the command type first, so
if it's write, we end up with req->data_len == 0 and direction as
DMA_TO_DEVICE which confuses some drivers)
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/scsi_lib.c

@@ -1215,12 +1215,12 @@ static int scsi_prep_fn(struct request_queue *q, struct request *req)
 		} else {
 			memcpy(cmd->cmnd, req->cmd, sizeof(cmd->cmnd));
 			cmd->cmd_len = req->cmd_len;
-			if (rq_data_dir(req) == WRITE)
+			if (!req->data_len)
+				cmd->sc_data_direction = DMA_NONE;
+			else if (rq_data_dir(req) == WRITE)
 				cmd->sc_data_direction = DMA_TO_DEVICE;
-			else if (req->data_len)
-				cmd->sc_data_direction = DMA_FROM_DEVICE;
 			else
-				cmd->sc_data_direction = DMA_NONE;
+				cmd->sc_data_direction = DMA_FROM_DEVICE;
 			
 			cmd->transfersize = req->data_len;
 			cmd->allowed = 3;