Bluetooth: mt7921s: fix btmtksdio_[drv|fw]_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 btmtksdio_mcu_[drv|fw]_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: c603bf1f ("Bluetooth: btmtksdio: add MT7921s Bluetooth support")
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Mark Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>