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: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/niu.c b/drivers/net/niu.c
index 5698c15..0c0b752 100644
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -8906,7 +8906,7 @@
 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,