mac80211: fix start_seq_num in Rx reorder offload

sta->last_seq_ctrl is the seq_ctrl field from the last header
seen, need to shift it 4 bits to extract the sequence number.
Otherwise the ieee80211_sn_less() check at the top of
ieee80211_sta_manage_reorder_buf drops frames until the sequence
number catches up.

Cc: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Denton Gentry <denton.gentry@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -1175,8 +1175,8 @@ static void ieee80211_iface_work(struct work_struct *work)
 			if (sta) {
 				u16 last_seq;
 
-				last_seq = le16_to_cpu(
-					sta->last_seq_ctrl[rx_agg->tid]);
+				last_seq = IEEE80211_SEQ_TO_SN(le16_to_cpu(
+					sta->last_seq_ctrl[rx_agg->tid]));
 
 				__ieee80211_start_rx_ba_session(sta,
 						0, 0,