ethernet/tile: use core min/max MTU checking

tilegx: min_mtu 68, max_mtu 1500 or 9000, depending on modparam
- remove tile_net_change_mtu now that it is fully redundant

tilepro: min_mtu 68, max_mtu 1500
- hardware supports jumbo packets up to 10226, but it's not implemented or
  tested yet, according to code comments

CC: netdev@vger.kernel.org
CC: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>