drivers: net: use skb_headlen()

replaces (skb->len - skb->data_len) occurrences by skb_headlen(skb)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tsi108_eth.c b/drivers/net/tsi108_eth.c
index 1292d23..a03730b 100644
--- a/drivers/net/tsi108_eth.c
+++ b/drivers/net/tsi108_eth.c
@@ -704,8 +704,8 @@
 
 		if (i == 0) {
 			data->txring[tx].buf0 = dma_map_single(NULL, skb->data,
-					skb->len - skb->data_len, DMA_TO_DEVICE);
-			data->txring[tx].len = skb->len - skb->data_len;
+					skb_headlen(skb), DMA_TO_DEVICE);
+			data->txring[tx].len = skb_headlen(skb);
 			misc |= TSI108_TX_SOF;
 		} else {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i - 1];