mt76: mt7615: unlock dfs bands

Unlock dfs channels for mt7615 devices since the driver supports
radar detection. Dfs pattern detector has been tested successfully by
mt7615 users.
Do not unlock DFS frequencies for mt7663 devices since it has not been
tested yet.

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