smsc95xx: add tx checksum offload support

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>