ath9k_hw: check if the chip failed to wake up

In an RFC patch, Sven Eckelmann and Simon Wunderlich reported:

"QCA 802.11n chips (especially AR9330/AR9340) sometimes end up in a
state in which a read of AR_CFG always returns 0xdeadbeef.
This should not happen when when the power_mode of the device is
ATH9K_PM_AWAKE."

Include the check for the default register state in the existing MAC
hang check.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

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

@@ -1603,6 +1603,10 @@ bool ath9k_hw_check_alive(struct ath_hw *ah)
 	int count = 50;
 	u32 reg, last_val;
 
+	/* Check if chip failed to wake up */
+	if (REG_READ(ah, AR_CFG) == 0xdeadbeef)
+		return false;
+
 	if (AR_SREV_9300(ah))
 		return !ath9k_hw_detect_mac_hang(ah);