Staging: batman-adv: splitting /proc vis file into vis_server and vis_data

The /proc vis file was used to enable/disable the vis server and to output
the vis data at the same time. This behaviour was confusing and lacked a
proper method to display the current vis server status.
This patch seperates the 2 functionalities:
* use vis_server to enable/disable the vis server and to retrieve its status
* use vis_data to retrieve the vis raw data (if the server is enabled)

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>