usb: dwc3: ep0: explicitly call dwc3_ep0_prepare_one_trb()

Let's call dwc3_ep0_prepare_one_trb() explicitly
because there are occasions where we will need more
than one TRB to handle an EP0 transfer.

A follow-up patch will fix one bug related to
multiple-TRB Data Phases when it comes to
mapping/unmapping requests for DMA.

Cc: <stable@vger.kernel.org>
Reported-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>