bcm63xx_enet: add xmit_more support

Support bulking hardware TX queue by using netdev_xmit_more().

Signed-off-by: Sieng Piaw Liew <liew.s.piaw@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bcm63xx_enet.c

@@ -634,6 +634,7 @@ bcm_enet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	netdev_sent_queue(dev, skb->len);
 
 	/* kick tx dma */
+	if (!netdev_xmit_more() || !priv->tx_desc_count)
 		enet_dmac_writel(priv, priv->dma_chan_en_mask,
 				 ENETDMAC_CHANCFG, priv->tx_chan);