batman-adv: main, Convert is_my_mac() to bool

It is much clearer to see a bool type as return value than 'int' for
functions that are supposed to return true or false.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>