mac80211: Assign a default mesh beaconing interval.

The mesh stack was enabling beaconing without specifying an interval.  This
patch defines a default beaconing interval of 1s.

Incidentally, this fixes mesh beaconing in mac80211_hwsim devices.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>