batman-adv: Add missing kerneldoc for extack

The parameter extack was added to batadv_softif_slave_add without adding
the kernel-doc for it. This caused kernel-doc warnings.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>