mt76: mt7615: implement probing and firmware loading on MT7622

MT7622 does not have a CR4 microcontroller, so it only uses its own N9
firmware.

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