spi: core: Fix check for dma_map_sg() failures

According to Documentation/DMA-API.txt, dma_map_sg() returns 0 on failure.
As spi_map_buf() returns an error code, convert zero into -ENOMEM.
Keep the existing check for negative numbers just in case.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi.c

@@ -619,6 +619,8 @@ static int spi_map_buf(struct spi_master *master, struct device *dev,
 	}
 
 	ret = dma_map_sg(dev, sgt->sgl, sgt->nents, dir);
+	if (!ret)
+		ret = -ENOMEM;
 	if (ret < 0) {
 		sg_free_table(sgt);
 		return ret;