commit cdcedd69 upstream.

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.
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>