ath9k_htc: Enable ANI in associated state only

While scanning, ANI is triggered unnecessarily where sta is in
unassociated state. And cancelling ani work in ath9k_htc_stop
is not required.

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