net: lantiq_etop: add missing comment for wmb()

This patch adds the missing code comment for memory barrier
call and fixes checkpatch warning:

WARNING: memory barrier without comment
+	wmb();

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://lore.kernel.org/r/20211228214910.70810-1-olek2@wp.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/lantiq_etop.c

@@ -498,6 +498,7 @@ ltq_etop_tx(struct sk_buff *skb, struct net_device *dev)
 	spin_lock_irqsave(&priv->lock, flags);
 	desc->addr = ((unsigned int) dma_map_single(&priv->pdev->dev, skb->data, len,
 						DMA_TO_DEVICE)) - byte_offset;
+	/* Make sure the address is written before we give it to HW */
 	wmb();
 	desc->ctl = LTQ_DMA_OWN | LTQ_DMA_SOP | LTQ_DMA_EOP |
 		LTQ_DMA_TX_OFFSET(byte_offset) | (len & LTQ_DMA_SIZE_MASK);