wcn36xx: cancel pending scan request when interface goes down

When the network interface goes down while a scan request is still
pending that can't be stopped due to firmware hickups, wcn->scan_req
remains set, even though the hardware is deinitialized. This results
in -EBUSY for all scan requests after the interface was brought up
again.

Fix this by explicitly completing pending scan requests in
wcn36xx_stop().

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>