nodewatcher: Remove leftover use of batman-adv sysfs

In d9ec8edb ("batman-adv: Move from DEBUGFS to batctl") we
switched from deprecated DEBUGFS to batctl.

Despite debugfs, batctl seems to also have deprecated sysfs
(https://www.open-mesh.org/news/90

).

This patch thus replaces a reference to iface_status by the
corresponding batctl command.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>