Documentation: DMA-API-HOWTO: Fix a typo

Fix a type in example variable name.
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DMA-API-HOWTO.txt

@@ -699,7 +699,7 @@ to use the dma_sync_*() interfaces.
 		dma_addr_t mapping;
 
 		mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
-		if (dma_mapping_error(cp->dev, dma_handle)) {
+		if (dma_mapping_error(cp->dev, mapping)) {
 			/*
 			 * reduce current DMA mapping usage,
 			 * delay and try again later or