ath9k: fix crash in ath_update_survey_stats

If ah->curchan is uninitialized, the channel index is bogus, which leads
to invalid memory access when the cycle counters are updated.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -182,6 +182,9 @@ static void ath_update_survey_stats(struct ath_softc *sc)
 	struct ath_cycle_counters *cc = &common->cc_survey;
 	unsigned int div = common->clockrate * 1000;
 
+	if (!ah->curchan)
+		return;
+
 	if (ah->power_mode == ATH9K_PM_AWAKE)
 		ath_hw_cycle_counters_update(common);