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>