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>