batman-adv: create helper function to get AP isolation status

The AP isolation status may be evaluated in different spots.
Create an helper function to avoid code duplication.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>

diff --cc net/batman-adv/main.c
Simple merge

diff --cc net/batman-adv/main.h
Simple merge

diff --cc net/batman-adv/translation-table.c
Simple merge