mac80211: allow re-open the blocked peer link in mesh

Peer link which is blocked using the "iw mesh0 station
set <MAC addr> plink_action block" is previously not able
to re-open using "iw mesh0 station set <MAC addr>
plink_action open". This patch is intended to solve this.

If the station plink state remains at OPN_SNT once open,
try block and open again should solve this problem.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>