Only write a success node when coming online, not going offline. Delete the

Only write a success node when coming online, not going offline.  Delete the
interface from the bridge when going offline -- the comment stating that
interfaces are auto-removed does not seem to be true, at least some of the
time.  Log failure of brctl delif and ifconfig down only at debug level, as
this script may be called more than once, depending upon the hotplug
configuration.

Signed-off-by: Ewan Mellor <ewan@xensource.com>