Bluetooth: btusb: mediatek: add callback function in btusb_disconnect

Add disconnect callback function in btusb_disconnect which is reserved
for vendor specific usage before deregister hci in btusb_disconnect.

Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>