drivers/net: fix sparse warning: returning void-valued expression

Fix this sparse warning:

  drivers/net/niu.c:8850:2: warning: returning void-valued expression
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/niu.c

@@ -8906,7 +8906,7 @@ static u64 niu_pci_map_page(struct device *dev, struct page *page,
 static void niu_pci_unmap_page(struct device *dev, u64 dma_address,
 			       size_t size, enum dma_data_direction direction)
 {
-	return dma_unmap_page(dev, dma_address, size, direction);
+	dma_unmap_page(dev, dma_address, size, direction);
 }
 
 static u64 niu_pci_map_single(struct device *dev, void *cpu_addr,