mac80211: fix STA_SLOW_THRESHOLD htmldocs failure

Patch fixes htmldocs build problem:

Error(.//net/mac80211/sta_info.h:416): cannot understand prototype: 'STA_SLOW_THRESHOLD 6000  '

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.h

@@ -398,7 +398,7 @@ struct ieee80211_sta_rx_stats {
 	u64 msdu[IEEE80211_NUM_TIDS + 1];
 };
 
-/**
+/*
  * The bandwidth threshold below which the per-station CoDel parameters will be
  * scaled to be more lenient (to prevent starvation of slow stations). This
  * value will be scaled by the number of active stations when it is being