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: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NMark Brown <broonie@linaro.org>