mt76: mt7615: fix throughput regression on DFS channels

For some reason, mt7615 reacts badly to repeatedly enabling/disabling the radar
detector without also switching the channel.
This results in very bad throughput on DFS channels, because
hw->conf.radar_enabled can get toggled a few times after CAC ends.
Fix this by always leaving the DFS detector enabled on DFS channels and instead
suppress unwanted detection events.

Fixes: 2c86f675 ("mt76: mt7615: fix/rewrite the dfs state handling logic")
Signed-off-by: Felix Fietkau <nbd@nbd.name>