[BRIDGE]: Do sysfs registration inside rtnl.

Now that netdevice sysfs registration is done as part of
register_netdevice; bridge code no longer has to be tricky when adding
it's kobjects to bridges.

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