Staging: batman-adv: convert vis_interval into define

vis_interval does not neccesarily needs to be a variable, as there is
no way to change it anyway (and probably no need to). We can therefore
remove yet another global variable.

Thanks Marek for pointing this out.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
[sven.eckelmann@gmx.de: Rework on top of current version]
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>