Staging: batman-adv: atomic variable for vis-srv activation

This fixes the bug discovered by Marek Lindner which did not allow
turning on the vis-server before an interface has been added. With this
patch we are using a global atomic variable for activating and
deactiating the vis-server-mode, which can be used before
inserting an interface.

Signed-off-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>