batman-adv: Set the txqueuelen to zero when creating soft interface

Like other virtual interfaces, e.g. br0, we don't need a transmit
queue. Packets should only be queued on real interfaces which are
underneath. In practice this patch makes little difference since the
virtual interfaces can accept packets as fast as they come, but the
patch will avoid bufferbloat questions to the mailling lists in the
future.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>