wcn36xx: Ensure finish scan is not requested before start scan

If the operating channel is the first in the scan list, it was seen that
a finish scan request would be sent before a start scan request was
sent, causing the firmware to fail all future scans. Track the current
channel being scanned to avoid requesting the scan finish before it
starts.

Cc: <stable@vger.kernel.org>
Fixes: 5973a294 ("wcn36xx: Fix software-driven scan")
Signed-off-by: Joseph Gates <jgates@squareup.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1629286303-13179-1-git-send-email-loic.poulain@linaro.org