wifi: mt76: mt7996: set correct background radar capability

Some of the variants do not support background radar, so add a helper
to report background radar capability.

Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/20240926032440.15978-3-shayne.chen@mediatek.com


Signed-off-by: Felix Fietkau <nbd@nbd.name>