batman-adv: fix and simplify condition when bonding should be used

The current condition actually does NOT consider bonding when the
interface the packet came in from is the soft interface, which is the
opposite of what it should do (and the comment describes). Fix that and
slightly simplify the condition.

Reported-by: Ray Gibson <booray@gmail.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>