mt76: mt7921: reduce the data latency during hw scan

Reduce the data latency during hw_scan by the split scan which would switch
back to operational channel right after scanning each channel done.

Suggested-by: Asda Wen <Asda.Wen@mediatek.com>
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>