Bluetooth: btusb: mediatek: add intf release flow when usb disconnect

MediaTek claim an special usb intr interface for ISO data transmission.
The interface need to be released before unregistering hci device when
usb disconnect. Removing BT usb dongle without properly releasing the
interface may cause Kernel panic while unregister hci device.

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