ath10k: move hw_scan worker queuing

The remain_on_channel callback needs different
timeout. Calling ieee80211_queue_work() with a
shorter delay after calling it with a longer delay
will not change the timer. This caused the
offchannel timeout worker to not trigger in time
and caused the device to stay on channel longer
then expected. This could cause some problems and
was be easily reproduced with `iw offchannel`
command.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>