wifi: mt76: mt7915: set correct background radar capability

Some of the variants do not support background radar, so add a
helper to report background radar capability.
For mt7916, only the variant of 5G 2T2R + 1R supports background
radar.

Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Reviewed-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20250320015909.3948612-1-StanleyYP.Wang@mediatek.com


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