net: NEWEMAC: Fix problem with mtu > 4080 on non TAH equipped 4xx PPC's

Currently, all non TAH equipped 4xx PPC's call emac_start_xmit() upon
xmit. This routine doesn't check if the frame length exceeds the max.
MAL buffer size.

This patch now changes the driver to call emac_start_xmit_sg() on all
GigE platforms and not only the TAH equipped ones (440GX). This enables
an MTU of 9000 instead 4080.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>