batman-adv: print the VID properly

Since the MSB bits of any vid variable are now used for
storing flags, print the vid properly by taking the flags
away and printing -1 in case of VID representing no real
VLAN.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>