usb: dwc3: gadget: don't prevent gadget from being probed if we fail

In case we fail our ->udc_start() callback, we
should be ready to accept another modprobe following
the failed one.

We had forgotten to clear dwc->gadget_driver back
to NULL and, because of that, we were preventing
gadget driver modprobe from being retried.

Cc: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>