ath9k: Disable autosleep feature by default.

Autosleep needs to be disabled for AR9287 chipsets also.
Since autosleep is not used for any of the currently supported
chipsets, disable it by default and can be enabled if needed
for any of the future chipsets.

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