Documentation: DMA-API: fix a function name of max_mapping_size

The exported function name is dma_max_mapping_size(), not
dma_direct_max_mapping_size() so that this patch fixes
the function name in the documentation.

Fixes: 133d624b ("dma: Introduce dma_max_mapping_size()")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DMA-API.txt

@@ -198,7 +198,7 @@ call to set the mask to the value returned.
 ::
 
 	size_t
-	dma_direct_max_mapping_size(struct device *dev);
+	dma_max_mapping_size(struct device *dev);
 
 Returns the maximum size of a mapping for the device. The size parameter
 of the mapping functions like dma_map_single(), dma_map_page() and