[BRIDGE]: limited ethtool support

Add limited ethtool support to bridge to allow disabling
features.

Note: if underlying device does not support a feature (like checksum
offload), then the bridge device won't inherit it.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>