wcn36xx: Implement cancel_hw_scan

In the even that the wcn36xx interface is brought down while a hw_scan
is active we must abort and wait for the ongoing scan to signal
completion to mac80211.

Reported-by: Mart Raudsepp <leio@gentoo.org>
Fixes: 88603903 ("wcn36xx: Implement firmware assisted scan")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>