wifi: mt76: mt7915: fix background radar event being blocked

The background radar uses MT_RX_SEL2 as its band indication, so fix it.

Fixes: 7a53eecd

 (wifi: mt76: mt7915: check the correctness of event data)
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>