batman-adv: Update wifi flags on upper link change

Things like VLANs don't have their link set when they are created. Thus
the wifi flags have to be evaluated later to fix their contents for the
link interface.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>