wcn36xx: Fix warning due to duplicate scan_completed notification

The wcn36xx_cancel_hw_scan method stops the hw scan and notify the
scan completion via ieee80211_scan_completed.
However, on scan offload cancellation, firmware sends a scan complete
indication, triggering a new call to ieee80211_scan_completed.
This leads to kernel warn since the scan has already been completed.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>