wil6210: delay remain on channel when scan is active

Currently it was possible to call remain_on_channel(ROC)
while scan was active and this caused a crash in the FW.
In order to fix this problem and make the behavior
consistent with other drivers, queue the ROC in case
a scan is active and try it again when scan is done.
As part of the fix, clean up some locking issues and
return error if scan is called while ROC is active.

Signed-off-by: Lior David <qca_liord@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>