mac80211: fix warning on htmldocs for last_tdls_pkt_time

Forgot to add an entry to the struct description of sta_info.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>