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>