usbnet: add new flag FLAG_NOARP for usb net devices

We do have some USB net devices, which cannot do ARP.
so we can introduce a new flag FLAG_NOARP, then client drivers
can easily handle this kind of devices

Signed-off-by: Wei Shuai <cpuwolf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>