ath9k_hw: Enable generic timer interrupt.

Generic timer interrupt was not triggered unless autosleep was
disabled. Since autosleep is enabled in the newer chipsets,
enable generic timer for using with bt coex logic.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>