usb: dwc3: ep0: tidy up Pending Request handling

The way our code was written, we should never have
a DWC3_EP_PENDING_REQUEST flag set out of a Data Phase
and the code in __dwc3_gadget_ep0_queue() did not
reflect that situation properly.

Tidy up that case to avoid any possible mistakes
when starting requests for IRQs which are long
gone.

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