batman-adv: Simplify gw_check_election(), use gw_get_selected()

gw_get_selected() can get us the desired orig_node directly, therefore
reusing that function in gw_check_election().

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>