mac80211: Fix documentation strings for airtime-related variables

There was a typo in the documentation for weight_multiplier in mac80211.h,
and the doc was missing entirely for airtime and airtime_weight in sta_info.h.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>