net: stmmac: do not clear TBS enable bit on link up/down

With the dma conf being reallocated on each call to stmmac_open(), any
information in there is lost, unless we specifically handle it.

The STMMAC_TBS_EN bit is set when adding an etf qdisc, and the etf qdisc
therefore would stop working when link was set down and then back up.

Fixes: ba39b344 ("net: ethernet: stmicro: stmmac: generate stmmac dma conf before open")
Cc: stable@vger.kernel.org
Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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

@@ -3932,6 +3932,9 @@ static int __stmmac_open(struct net_device *dev,
 	priv->rx_copybreak = STMMAC_RX_COPYBREAK;
 
 	buf_sz = dma_conf->dma_buf_sz;
+	for (int i = 0; i < MTL_MAX_TX_QUEUES; i++)
+		if (priv->dma_conf.tx_queue[i].tbs & STMMAC_TBS_EN)
+			dma_conf->tx_queue[i].tbs = priv->dma_conf.tx_queue[i].tbs;
 	memcpy(&priv->dma_conf, dma_conf, sizeof(*dma_conf));
 
 	stmmac_reset_queues_param(priv);