mac80211: mesh: factor out common plink close/estab code

Reject and accepted close events always put the host in the
holding state and compute a reason code based only on the
current state.  Likewise on establish we always do the same
setup.  Put these in functions to save some duplicated code.

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