ath9k: Fix interrupt statistics

Initialize "sync_cause" to zero since commit
"ath9k: move ath9k_debug_sync_cause out of ath9k_hw"
fills it conditionally based on ISR status.
Not doing this results in garbage values in debugfs.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -544,7 +544,7 @@ irqreturn_t ath_isr(int irq, void *dev)
 	struct ath_hw *ah = sc->sc_ah;
 	struct ath_common *common = ath9k_hw_common(ah);
 	enum ath9k_int status;
-	u32 sync_cause;
+	u32 sync_cause = 0;
 	bool sched = false;
 
 	/*