usb: dwc3: gadget: Enable XferComplete event

To switch from one stream to another, this requires the driver to start
a new transfer with a specific stream ID. For a transfer to complete,
the driver needs to indicate the last TRB of a transfer, and it needs to
enable XferComplete event to handle completed TRBs of a transfer. Let's
enable this event only for stream capable endpoints.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>