This patch reverts: https://github.com/torvalds/linux/commit/eaf4fac478077d4ed57cbca2c044c4b58a96bd98

It works around following issues:
 - no way to change MTU (tx_fifo_size is reported as 0 for Rockchip's dwmac)

Signed-off-by: Piotr Szczepanik <piter75@gmail.com>

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 122a06972..951d2281f 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -3961,25 +3961,13 @@ static void stmmac_set_rx_mode(struct net_device *dev)
 static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 {
 	struct stmmac_priv *priv = netdev_priv(dev);
-	int txfifosz = priv->plat->tx_fifo_size;
 	const int mtu = new_mtu;
-
-	if (txfifosz == 0)
-		txfifosz = priv->dma_cap.tx_fifo_size;
-
-	txfifosz /= priv->plat->tx_queues_to_use;
 
 	if (netif_running(dev)) {
 		netdev_err(priv->dev, "must be stopped to change its MTU\n");
 		return -EBUSY;
 	}
 
-	new_mtu = STMMAC_ALIGN(new_mtu);
-
-	/* If condition true, FIFO is too small or MTU too large */
-	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
-		return -EINVAL;
-
 	dev->mtu = mtu;
 
 	netdev_update_features(dev);