mac80211: fix oper channel timestamp updation

Whenever the station informs the AP that it is about to leave the
operating channel, the timestamp should be recorded. It is handled
in scan resume but not in scan start. Fix that.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>