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

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

The commit 9799c50372b2 ("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>