ftgmac100: Fix build.

drivers/net/ethernet/faraday/ftgmac100.c:777:13: error: 'skb_frag_t {aka struct bio_vec}' has no member named 'size'

Fallout from the skb_frag_t conversion to bio_vec, simply
use skb_frag_size().

Fixes: b8b576a1 ("net: Rename skb_frag_t size to bv_len")
Reported-by: René van Dorst <opensource@vdorst.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/faraday/ftgmac100.c

@@ -774,7 +774,7 @@ static netdev_tx_t ftgmac100_hard_start_xmit(struct sk_buff *skb,
 	for (i = 0; i < nfrags; i++) {
 		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
-		len = frag->size;
+		len = skb_frag_size(frag);
 
 		/* Map it */
 		map = skb_frag_dma_map(priv->dev, frag, 0, len,