ath9k: setup correct skb priority for nullfunc

After queue nullfunc for MCC case, we hit WARN_ON
in xmit.c:2398 while skb priority wasn't set.
Signed-off-by: NJanusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath9k/channel.c

@@ -1101,6 +1101,7 @@ ath_chanctx_send_vif_ps_frame(struct ath_softc *sc, struct ath_vif *avp,
 			nullfunc->frame_control |=
 				cpu_to_le16(IEEE80211_FCTL_PM);
 
+		skb->priority = 7;
 		skb_set_queue_mapping(skb, IEEE80211_AC_VO);
 		if (!ieee80211_tx_prepare_skb(sc->hw, vif, skb, band, &sta)) {
 			dev_kfree_skb_any(skb);