carl9170: fix signal strength reporting issues

On A-MPDU frames, the hardware only reports valid
signal strength data for the last subframe.

This patch fixes it by flagging everything but the
last subframe in an A-MPDU to tell mac80211 to
ignore the signal strength entirely. Otherwise
the empty value (= 0 dbm) will distort the
average quite badly.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/carl9170/rx.c

@@ -814,6 +814,8 @@ static void carl9170_rx_untie_data(struct ar9170 *ar, u8 *buf, int len)
 
 	if (phy)
 		carl9170_rx_phy_status(ar, phy, &status);
+	else
+		status.flag |= RX_FLAG_NO_SIGNAL_VAL;
 
 	if (carl9170_handle_mpdu(ar, buf, mpdu_len, &status))
 		goto drop;