myri10ge: correctly detect when TSO should be used

Correctly detect when TSO should be used on transmit by looking at the
skb->gso_size rather than seeing if the frame was larger than our MTU.
The old method causes problems when a host with a large (jumbo) MTU is
sending to a host with a small (standard) MTU.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>