stmmac: release tx lock, in case of dma mapping error.

Add missing spin_unlock when tx frames gets dropped.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2026,6 +2026,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	return NETDEV_TX_OK;
 
 dma_map_err:
+	spin_unlock(&priv->tx_lock);
 	dev_err(priv->device, "Tx dma map failed\n");
 	dev_kfree_skb(skb);
 	priv->dev->stats.tx_dropped++;