ethernet/ibm: use core min/max MTU checking

ehea: min_mtu 68, max_mtu 9022
- remove ehea_change_mtu, it's now redundant

emac: min_mtu 46, max_mtu 1500 or whatever gets read from OF

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