Bluetooth: btusb: fix up firmware download sequence

Data RAM on the device have to be powered on before starting to download
the firmware.

Fixes: a1c49c43

 ("Bluetooth: btusb: Add protocol support for MediaTek MT7668U USB devices")
Co-developed-by: Mark Chen <Mark-YW.Chen@mediatek.com>
Signed-off-by: Mark Chen <Mark-YW.Chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>