wcn36xx: Fix warning due to duplicate scan_completed notification

wcn36xx: Fix warning due to duplicate scan_completed notification

BugLink: http://bugs.launchpad.net/bugs/1763040
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>
(cherry picked from commit 1dbde8e6bd6c4780273a1f387f9e783edd37c845
http://git.linaro.org/landing-teams/working/qualcomm/kernel.git release/qcomlt-4.14)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>