batman-adv: Drop reduntant batadv interface check

If batadv_hardif_enable_interface is called then its called from its
callback ndo_add_slave. It is therefore not necessary to check if it is a
batadv interface.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>