ethernet/dlink: use core min/max MTU checking

dl2k: min_mtu 68, max_mtu 1536 or 8000, depending on hardware
- Removed change_mtu, does nothing productive anymore

sundance: min_mtu 68, max_mtu 8191

CC: netdev@vger.kernel.org
CC: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>