mac80211: remove writable debugs mesh parameters

These parameters shouldn't be configurable via debugfs, if they
need to be configurable nl80211 support has to be added, if not
then they don't need to be writable here either.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Javier Cardona <javier@cozybit.com>
Cc: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>