mac80211: Fix addition of mesh configuration element

The code was setting the capabilities byte to zero,
after it was already properly set previously. Fix it.

The bug was found while debugging hwsim mesh tests failures
that happened since the commit mentioned below.

Fixes: 76f43b4c ("mac80211: Remove invalid flag operations in mesh TSF synchronization")
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>