enetc: Fix NULL dma address unmap for Tx BD extensions

For the unlikely case of TxBD extensions (i.e. ptp)
the driver tries to unmap the tx_swbd corresponding
to the extension, which is bogus as it has no buffer
attached.
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/enetc/enetc.c

@@ -313,7 +313,9 @@ static bool enetc_clean_tx_ring(struct enetc_bdr *tx_ring, int napi_budget)
 	while (bds_to_clean && tx_frm_cnt < ENETC_DEFAULT_TX_WORK) {
 		bool is_eof = !!tx_swbd->skb;
 
-		enetc_unmap_tx_buff(tx_ring, tx_swbd);
+		if (likely(tx_swbd->dma))
+			enetc_unmap_tx_buff(tx_ring, tx_swbd);
+
 		if (is_eof) {
 			napi_consume_skb(tx_swbd->skb, napi_budget);
 			tx_swbd->skb = NULL;