wifi: mt76: mt7996: use correct phy for background radar event

If driver directly uses the band_idx reported from the radar event to
access mt76_phy array, it will get the wrong phy for background radar.
Fix this by adjusting the statement.

Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>