Revert "dma-mapping: clear dev->dma_ops in arch_teardown_dma_ops"

This reverts commit 46053c73685411915d3de50c5a0045beef32806b.

This change breaks architectures setting up dma_ops in their own magic
way and not using arch_setup_dma_ops, so revert it.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index bd81e74..d23fc45 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -665,10 +665,7 @@
 #endif
 
 #ifndef arch_teardown_dma_ops
-static inline void arch_teardown_dma_ops(struct device *dev)
-{
-	dev->dma_ops = NULL;
-}
+static inline void arch_teardown_dma_ops(struct device *dev) { }
 #endif
 
 static inline unsigned int dma_get_max_seg_size(struct device *dev)