mt76: testmode: move mtd part to mt76_dev

Move testmode mtd variables to mt76_dev, since they are the same on
each phy.

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>