ath9k: avoid enabling interrupts while processing rx

The assumsion is that while processing ath9k tasklet,
interrupts were already disabled and it will be enabled
at the completion of ath9k tasklet. But whenever TSFOOR is raised,
the driver configures the beacon timers after having received a
beacon frame from the AP which inturn enables the interrupts.

Cc: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>