netdev: Don't cache network device features.

netdev: Don't cache network device features.

The new implementation of secchan wants to get updates of network device
features by keeping a network device open for each port and checking its
features when notified of a port status change.  This wouldn't work, since
the features were cached once at startup.  This commit makes the netdev
code check the actual devices features on each call.

Also, generalizes do_ethtool() to be useful for other kinds of ethtool
operations.