xgbe: convert to new udp_tunnel_nic infra

Make use of the new udp_tunnel_nic infra. Don't clear the features
when VxLAN port is not present to make all drivers behave the same.
Driver will now (until we address the problem in the core) leave
the RX UDP tunnel feature always on, since this is what most drivers
do.

Remove the list of VxLAN ports, just program the one core told us to.

The driver seem to want to clear the VxLAN ports on close but it
doesn't seem to flush the port list properly so it'd get wrong
use counts after close/open. Again since it calls its own open
handler we need the reset notification hack.

v2:
 - fix kbuild warning

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>