MIPS: use dma_direct_ops for coherent I/O

Switch the simple cache coherent architectures that don't require any
DMA address translation to dma_direct_ops.

We'll soon use at least parts of the direct DMA ops implementation for
all platforms, so select the symbol globally.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Patchwork: https://patchwork.linux-mips.org/patch/19540/
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Tom Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: iommu@lists.linux-foundation.org
Cc: linux-mips@linux-mips.org

arch/mips/include/asm/dma-mapping.h

@@ -20,7 +20,7 @@ static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
 #elif defined(CONFIG_MIPS_DMA_DEFAULT)
 	return &mips_default_dma_map_ops;
 #else
-	return NULL;
+	return &dma_direct_ops;
 #endif
 }