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/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 2c9da6b..0d4155a 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -2018,6 +2018,9 @@
 
 	ath_dbg(common, XMIT, "TX complete: skb: %p\n", skb);
 
+	if (sc->sc_ah->caldata)
+		sc->sc_ah->caldata->paprd_packet_sent = true;
+
 	if (!(tx_flags & ATH_TX_ERROR))
 		/* Frame was ACKed */
 		tx_info->flags |= IEEE80211_TX_STAT_ACK;