[AVR32] Implement dma_mapping_error()

dma_map_single() never fails, so dma_mapping_error() simply returns 0.
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

include/asm-avr32/dma-mapping.h

@@ -32,6 +32,14 @@ static inline int dma_set_mask(struct device *dev, u64 dma_mask)
 	return 0;
 }
 
+/*
+ * dma_map_single can't fail as it is implemented now.
+ */
+static inline int dma_mapping_error(dma_addr_t addr)
+{
+	return 0;
+}
+
 /**
  * dma_alloc_coherent - allocate consistent memory for DMA
  * @dev: valid struct device pointer, or NULL for ISA and EISA-like devices