wifi: mt76: mt7996: add sanity checks for background radar trigger

Check if background radar is enabled or not before manually triggering it,
and also add more checks in radar detected event.

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>