batman-adv: remove returns at the end of void functions

Return at the end of void functions is not needed.

Since most of the void functions in the code do not do so,
make all the others consistent by removing the useless
returns. Actually all the functions to be "fixed" are in
network-coding.h only.

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