wifi: mac80211: add documentation for amsdu_mesh_control

This documentation wasn't added in the original patch,
add it now.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 6e4c0d04 ("wifi: mac80211: add a workaround for receiving non-standard mesh A-MSDU")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.h

@@ -622,6 +622,8 @@ struct link_sta_info {
  *	taken from HT/VHT capabilities or VHT operating mode notification
  * @cparams: CoDel parameters for this station.
  * @reserved_tid: reserved TID (if any, otherwise IEEE80211_TID_UNRESERVED)
+ * @amsdu_mesh_control: track the mesh A-MSDU format used by the peer
+ *	(-1: not yet known, 0: non-standard [without mesh header], 1: standard)
  * @fast_tx: TX fastpath information
  * @fast_rx: RX fastpath information
  * @tdls_chandef: a TDLS peer can have a wider chandef that is compatible to