mmc: renesas_sdhi: fix kernel panic in _internal_dmac.c

Since this driver checks if the return value of dma_map_sg() is minus
or not and keeps to enable the DMAC, it may cause kernel panic when
the dma_map_sg() returns 0. So, this patch fixes the issue.

Reported-by: Dirk Behme <[e-mail]>
Fixes: 2a68ea7896e3 ("mmc: renesas-sdhi: add support for R-Car Gen3 SDHI DMAC")
Signed-off-by: Yoshihiro Shimoda <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Wolfram Sang <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/renesas_sdhi_internal_dmac.c

···
 	WARN_ON(host->sg_len > 1);
 
 	/* This DMAC cannot handle if buffer is not 8-bytes alignment */
-	if (!IS_ALIGNED(sg->offset, 8)) {
-		host->force_pio = true;
-		renesas_sdhi_internal_dmac_enable_dma(host, false);
-		return;
-	}
+	if (!IS_ALIGNED(sg->offset, 8))
+		goto force_pio;
 
 	if (data->flags & MMC_DATA_READ) {
 		dtran_mode |= DTRAN_MODE_CH_NUM_CH1;
···
 	}
 
 	ret = dma_map_sg(&host->pdev->dev, sg, host->sg_len, dir);
-	if (ret < 0)
-		return;
+	if (ret == 0)
+		goto force_pio;
 
 	renesas_sdhi_internal_dmac_enable_dma(host, true);
 
···
 					    dtran_mode);
 	renesas_sdhi_internal_dmac_dm_write(host, DM_DTRAN_ADDR,
 					    sg->dma_address);
+
+	return;
+
+force_pio:
+	host->force_pio = true;
+	renesas_sdhi_internal_dmac_enable_dma(host, false);
 }
 
 static void renesas_sdhi_internal_dmac_issue_tasklet_fn(unsigned long arg)