batman-adv: Revert "fix splat on disabling an interface"

The commit 9799c503 ("batman-adv: fix splat on disabling an interface")
fixed a warning but at the same time broke the rtnl function add_slave for
devices which were temporarily removed.

batadv_softif_slave_add requires soft_iface of and hard_iface to be NULL
before it is allowed to be enslaved. But this resetting of soft_iface to
NULL in batadv_hardif_disable_interface was removed with the aforementioned
commit.
Reported-by: Julian Labus <julian@freifunk-rtk.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/hard-interface.c

@@ -652,6 +652,7 @@ void batadv_hardif_disable_interface(struct batadv_hard_iface *hard_iface,
 			batadv_softif_destroy_sysfs(hard_iface->soft_iface);
 	}
 
+	hard_iface->soft_iface = NULL;
 	batadv_hardif_put(hard_iface);
 
 out: