ath9k: No need to abort Rx path when autosleep is enabled.

For chipsets supporting autosleep feature, there is no need to abort
Rx engine since they are capable of automatically going back to sleep
after receiving a packet.

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