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>