batman-adv: main, batadv_compare_eth return bool

Declare the returntype of batadv_compare_eth as bool.
The function called inside this helper function
(ether_addr_equal_unaligned) also uses bool as return value, so there is
no need to return int.

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