mt76: mt7915: fix/rewrite the dfs state handling logic

Client mode on DFS channels was broken, because the old code was activating
the DFS detector on radar channels while leaving it in CAC state.
This was caused by making the decision based on the channel radar flag,
instead of hw->conf.radar_enabled.
In order to properly deal with the various corner cases, rip out the state
handling code and replace it with something that's much easier to reason
about.

Tested-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>