Bluetooth: btmtksdio: fix possible FW initialization failure

According to FW advised sequence, mt7921s need to re-acquire privilege
immediately after the firmware download is complete before normal running.
Otherwise, it is still possible the bus may be stuck in an abnormal status
that causes FW initialization failure in the current driver.

Fixes: 752aea58 ("Bluetooth: mt7921s: fix bus hang with wrong privilege")
Co-developed-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>