[BRIDGE]: Use ethtool to get port speed.

[BRIDGE]: Use ethtool to get port speed.

The bridge code needs to keep track of a cost estimate for each
port in the bridge.  Instead of a hack based on device name, try
and use ethtool to get port speed from device.  This has been tested
on e100 (uses ethtool_ops) and e1000 (does ethtool the hard way)
and dummy (no ethtool).

Need to export dev_ethtool() to allow bridge module to get to
it easily.

Code takes care to maintain same locking and semantics as if ioctl
was being done from application.