mlx4: min_mtu 46, max_mtu depends on hardware

mlx5: min_mtu 68, max_mtu depends on hardware

CC: netdev@vger.kernel.org
CC: Tariq Toukan <tariqt@mellanox.com>
CC: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>