mac80211: fix ampdu_action tx_start ssn

The start_seq_num is taken from the station's tid_seq[tid].
This is fine, except tid_seq sequence counter is shifted
by 4 bits to accommodate for frame fragmentation.

Both (iwlagn & ath9k) were unaffected by this minor glitch,
because they don't read the *ssn for the AMPDU_TX_START action.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/agg-tx.c

@@ -301,7 +301,7 @@ int ieee80211_start_tx_ba_session(struct ieee80211_sta *pubsta, u16 tid)
 	 * call back right away, it must see that the flow has begun */
 	*state |= HT_ADDBA_REQUESTED_MSK;
 
-	start_seq_num = sta->tid_seq[tid];
+	start_seq_num = sta->tid_seq[tid] >> 4;
 
 	ret = drv_ampdu_action(local, sdata, IEEE80211_AMPDU_TX_START,
 			       pubsta, tid, &start_seq_num);