mac80211: mesh: drop constant field mean_chain_len

The mean_chain_len field in struct mesh_table is copied whenever a
new mesh table is allocated, but only ever has the value 2 and is
never otherwise updated, so just remove it and use the related
define instead.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>