LAN9500 supports tx checksum offload, which slightly decreases cpu
utilisation.  The benefit isn't very large because we still require
the skb to be linearized, but it does save a few cycles.

This patch adds support for it, and enables it by default.
Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>