mt76: stop the radar detector after leaving dfs channel

The radar detctor is used for dfs channel. So it will start after switching
to dfs channel and will stop after leaving. The TX will be blocked if radar
detctor isn't stopped in non-dfs channel.

This patch resets the dfs state to indicate the radar detector needs to be
stopped.

Signed-off-by: Xing Song <xing.song@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>