mac80211: Suppress NEW_PEER_CANDIDATE event if no room

Previously, kernel sends NEW_PEER_CANDIDATE event to user land even if
the found peer does not have any room to accept other peer. This causes
continuous connection trials.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>