M68k only provides the arch_dma_prep_coherent symbol when an mmu is
enabled and not on the coldfire platform.  Fix the Kconfig symbol
selection up to match this.
Fixes: 69878ef47562 ("m68k: Implement arch_dma_prep_coherent()")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
        default y
        select ARCH_32BIT_OFF_T
        select ARCH_HAS_DMA_MMAP_PGPROT if MMU && !COLDFIRE
-       select ARCH_HAS_DMA_PREP_COHERENT
+       select ARCH_HAS_DMA_PREP_COHERENT if HAS_DMA && MMU && !COLDFIRE
        select ARCH_HAS_SYNC_DMA_FOR_DEVICE if HAS_DMA
        select ARCH_MIGHT_HAVE_PC_PARPORT if ISA
        select ARCH_NO_COHERENT_DMA_MMAP if !MMU