mac80211: fix off-by-one in llid check.

According to IEEE 802.11-2012 (8.4.2.104), no peering
management element exists with length 7. This code is checking
to see if llid is present to ignore close frames with different
llid, which would be IEs with length 8.

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