ehea: Simplify ehea_xmit2 and ehea_xmit3

Based on a patch from Michael Ellerman, clean up a significant
portion of the transmit path. There was a lot of duplication here.
Even worse, we were always checksumming tx packets and ignoring the
skb->ip_summed field.

Also remove NETIF_F_FRAGLIST from dev->features, I'm not sure why
it was enabled.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>