ethernet/sun: use core min/max MTU checking

cassini: min_mtu 60, max_mtu 9000

niu: min_mtu 68, max_mtu 9216

sungem: min_mtu 68, max_mtu 1500 (comments say jumbo mode is broken)

sunvnet: min_mtu 68, max_mtu 65535
- removed sunvnet_change_mut_common as it does nothing now

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