mt76: only stop tx queues on offchannel, not during the entire scan

During scans, mac80211 frequently switches back to the home channel to
minimize interruption of ongoing traffic. Keep regular tx queues active
during that time.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>