batman-adv: use batctl for configuration of gw_mode

batman-adv: use batctl for configuration of gw_mode

The batctl tool should be used for modifying settings in the batman-adv
driver to keep the scripts consistent and allow to use the correct way of
communicating with the kernel (sysfs vs. netlink).

The gw_mode, gw_bandwidth and gw_sel_class options are all handled by
the same gw_mode command in batctl and thus have to be combined depending
on the "gw_mode" value.

Signed-off-by: Sven Eckelmann <sven@narfation.org>