mt76: testmode: switch ib and wb rssi to array type for per-antenna report

Change ib_rssi and wb_rssi into array type, since they could be reported
by per-antenna.

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