mac80211: implement the meshconf formation info field

The Mesh Configuration Formation Info field contains the number of
neighbors.  This means that the beacon must be updated every time a
peer joins or leaves.

Signed-off-by: Rui Paulo <rpaulo@gmail.com>
Signed-off-by: Javier Cardona <rpaulo@gmail.com>
Reviewed-by: Andrey Yurovsky <andrey@cozybit.com>
Tested-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>