batman-adv: bcast: queue per interface, if needed

Currently we schedule a broadcast packet like:

3x: [ [(re-)queue] --> for(hard-if): maybe-transmit ]

The intention of queueing a broadcast packet multiple times is to
increase robustness for wireless interfaces. However on interfaces
which we only broadcast on once the queueing induces an unnecessary
penalty. This patch restructures the queueing to be performed on a per
interface basis:

for(hard-if):
- transmit
- if wireless: [queue] --> transmit --> [requeue] --> transmit

Next to the performance benefits on non-wireless interfaces this
should also make it easier to apply alternative strategies for
transmissions on wireless interfaces in the future (for instance sending
via unicast transmissions on wireless interfaces, without queueing in
batman-adv, if appropriate).

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>