wifi: mt76: connac: add support to load firmware for mt7990

wifi: mt76: connac: add support to load firmware for mt7990

Add firmware download support. Note that mt7990 does not have WA and DSP
firmwares. This is a preliminary patch to support mt7990 chipset.

Co-developed-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20250409140750.724437-3-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>