ehea: Remove NETIF_F_LLTX

Remove the deprecated NETIF_F_LLTX feature. Since the network stack
now provides the locking we can remove the driver specific
pr->xmit_lock.

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>