net: bcmgenet: return NULL instead of plain integer

Fixes the following sparse warning:

drivers/net/ethernet/broadcom/genet/bcmgenet.c:1351:16: warning:
 Using plain integer as NULL pointer
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/genet/bcmgenet.c

@@ -1348,7 +1348,7 @@ static struct sk_buff *bcmgenet_free_tx_cb(struct device *dev,
 		dma_unmap_addr_set(cb, dma_addr, 0);
 	}
 
-	return 0;
+	return NULL;
 }
 
 /* Simple helper to free a receive control block's resources */