dummy: expend mtu range for dummy device

After commit 61e84623

 ("net: centralize net_device min/max MTU checking"),
the mtu range for dummy device becomes [68, 1500].

This patch extends it to [0, 65535].

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