batman-adv: move GW mode and selection class to private data structure

To reduce the field pollution in our main batadv_priv data structure
we've already created some substructures so that we could group fields
in a convenient manner.

However gw_mode and gw_sel_class are still part of the main object.

More both fields to the GW private substructure.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>