wcn36xx: handle scan cancellation when firmware support is missing

For firmwares that don't have the SCAN_OFFLOAD feature bit set, do
not call into wcn36xx_smd_stop_hw_scan(). Instead, stop the asynchronous
work and call into ieee80211_scan_completed() immediately.

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