nlmon: use core MTU range checking in nlmon driver

Since commit 61e84623 ("net: centralize net_device min/max MTU checking"),
mtu range is checked at dev_set_mtu().

This patch adds min_mtu for nlmon device and remove unnecessary
ndo_change_mtu() function.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>