mt76: testmode: introduce dbdc support

Add testmode support for DBDC NICs (both MT7615D and MT7915D work).
Testmode data and parameters are moved from per-dev to per-phy
for maintaining the value of each band.

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>