ethernet/realtek: use core min/max MTU checking

8139cp: min_mtu 60, max_mtu 4096

8139too: min_mtu 68, max_mtu 1770

r8169: min_mtu 60, max_mtu depends on chipset, 1500 to 9k-ish

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