ath9k: Remove debug print in ISR

There is no need to do this and we can avoid
an unused variable warning when CONFIG_ATH9K_WOW is
not selected.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>