Fix software padding processing in Ethernet Tx path.

It's found that 20bytes(LLC), 28bytes, and 36bytes(PPP LCP Configuration
Ack) packets with additional 4bytes special tag for dsa driver would lead
to switch Tx CRC error, followed by switch output queue stuck issue.
Therefore, Ethernet driver check skb->len <= 40(36bytes PPP LCP + 4bytes
special tag) and conduct software padding.

Moreover, skb linearization should be checked again after conduct
software padding.

If without this patch, users might encounter problems when establishing
a PPPoE tunnel.

Change-Id: Id28fa3870dc9e905207d95634fe5cd692f363761
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/9129634

frank-w: changed condition to MTK_MIN_TX_LENGTH

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -1565,10 +1565,11 @@ static int mtk_tx_map(struct sk_buff *skb, struct net_device *dev,
 	int queue = skb_get_queue_mapping(skb);
 	int k = 0;
 
-	if (skb->len < 32) {
+	if (skb->len <= MTK_MIN_TX_LENGTH) {
 		if (skb_put_padto(skb, MTK_MIN_TX_LENGTH))
 			return -ENOMEM;
 
+		txd_info.last = !skb_is_nonlinear(skb);
 		txd_info.size = skb_headlen(skb);
 	}