usbnet & cdc-ether: Autosuspend for online devices

Using remote wakeup and delayed transmission to allow
online device to go into usb autosuspend.
Minimal alternate support for devices that don't support
remote wakeup.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David S. Miller <davem@davemloft.net>