mt76: mt7921s: fix mt7921s_mcu_[fw|drv]_pmctrl

According to the firmware behavior (even the oldest one in linux-firmware)
If the firmware is downloaded, MT7921S must rely on the additional mailbox
mechanism that resides in firmware to check if the device is the right
state for mt7921s_mcu_[fw|drv]_pmctrl. Otherwise, we still apply the old
way for that.

That is a necessary patch before we enable runtime pm for mt7921s as
default.

Fixes: 48fab5bb

 ("mt76: mt7921: introduce mt7921s support")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Leon Yen <leon.yen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>