ftgmac100: Add support for fragmented tx

Add NETIF_F_SG and create multiple TX ring entries for skb fragments.

On reclaim, the skb is only freed on the segment marked as "last".

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>