This is only used for testing for now. In the future it will be possible to
enable the second PHY through an proper API via device tree

Running:
 echo 1 > /sys/kernel/debug/ieee80211/phyX/mt76/dbdc

will register a second wiphy (and limit the primary one to 2 spatial streams).
The second wiphy will only be able to run on 5 GHz, while the primary one can
operate in both bands (if supported by the device).
Signed-off-by: NFelix Fietkau <nbd@nbd.name>