m68k: Sort out !CONFIG_MMU_SUN3 vs. CONFIG_HAS_DMA

In two places, we check !CONFIG_MMU_SUN3 while we should check
CONFIG_HAS_DMA instead.
While fixing this, the check in <asm/dma-mapping.h> became redundant
(<linux/dma-mapping.h> already handles this case), so just remove it.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>