batman-adv: netlink: reduce duplicate code by returning interfaces

Reduce duplicate code by using netlink helpers which return the
soft/hard interface directly. Instead of returning an interface index
which we are typically not interested in.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>