spi: core: Protect DMA code by #ifdef CONFIG_HAS_DMA

spi: core: Protect DMA code by #ifdef CONFIG_HAS_DMA

If NO_DMA=y:

drivers/built-in.o: In function `spi_map_buf':
spi.c:(.text+0x21bc60): undefined reference to `dma_map_sg'
drivers/built-in.o: In function `spi_unmap_buf.isra.33':
spi.c:(.text+0x21c32e): undefined reference to `dma_unmap_sg'
make[3]: *** [vmlinux] Error 1

Protect the DMA code by #ifdef CONFIG_HAS_DMA to fix this:
  - Extract __spi_map_msg() from spi_map_msg(),
  - Provide dummy definitions of __spi_map_msg() and spi_unmap_msg() if
    !CONFIG_HAS_DMA.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>