ledtrig-netdev: add support for renamed devices

The ppp0 interface is renamed after the connection is established. Due
to a missing NETDEV_REGISTER event, the ledtrig-netdev isn't aware of
the renamed interface and literally ignores the device
(no tx/rx indication, led isn't switched off with 'ifdown wan').

Signed-off-by: Mathias Kresin <openwrt@kresin.me>

SVN-Revision: 48048