batman-adv: Avoid precedence issues in macros

It must be avoided that arguments to a macro are evaluated ungrouped (which
enforces normal operator precendence). Otherwise the result of the macro
is not well defined.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>