batman-adv: Use bitwise instead of arithmetic operator for flags

This silences the following coccinelle warning:

"WARNING: sum of probable bitmasks, consider |"

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/multicast.c

@@ -528,7 +528,7 @@ update:
 	}
 
 	return !(mcast_data.flags &
-		 (BATADV_MCAST_WANT_ALL_IPV4 + BATADV_MCAST_WANT_ALL_IPV6));
+		 (BATADV_MCAST_WANT_ALL_IPV4 | BATADV_MCAST_WANT_ALL_IPV6));
 }
 
 /**