mt76: mt7921: fix the dwell time control

dwell time for the scan is not configurable according to the current
firmware submitted into linux-firmware.git, so leave the dwell time 0 to
indicate the dwell time always determined by the firmware.

Fixes: 399090ef ("mt76: mt76_connac: move hw_scan and sched_scan routine in mt76_connac_mcu module")
Suggested-by: Soul Huang <Soul.Huang@mediatek.com>
Co-developed-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: YN Chen <YN.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>