ath9k: fix PA linearization calibration related crash

Before PAPRD training can run, the card needs to have sent a packet for
thermal calibration. Sending a dummy packet with the PAPRD training flag
set causes a crash under some circumstance.
Fix the code by replacing the dummy tx with a delay that waits for a
real packet tx to have occurred.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/link.c b/drivers/net/wireless/ath/ath9k/link.c
index 1c24192..825a29c 100644
--- a/drivers/net/wireless/ath/ath9k/link.c
+++ b/drivers/net/wireless/ath/ath9k/link.c
@@ -256,7 +256,7 @@
 	int len = 1800;
 	int ret;
 
-	if (!caldata)
+	if (!caldata || !caldata->paprd_packet_sent || caldata->paprd_done)
 		return;
 
 	ath9k_ps_wakeup(sc);
@@ -283,13 +283,6 @@
 			continue;
 
 		chain_ok = 0;
-
-		ath_dbg(common, CALIBRATE,
-			"Sending PAPRD frame for thermal measurement on chain %d\n",
-			chain);
-		if (!ath_paprd_send_frame(sc, skb, chain))
-			goto fail_paprd;
-
 		ar9003_paprd_setup_gain_table(ah, chain);
 
 		ath_dbg(common, CALIBRATE,