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>