usb: dwc3: gadget: clear stall when disabling endpoint

so it seems like DWC3 IP doesn't clear stalls
automatically when we disable an endpoint, because
of that, we _must_ make sure stalls are cleared
before clearing the proper bit in DALEPENA register.

Cc: <stable@vger.kernel.org> # v3.4+
Reported-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>