usb: dwc3: gadget: simplify dwc3_prepare_one_trb()

We are already passing struct dwc3_request * to dwc3_prepare_one_trb(),
because of that there's no need to extract dma address and length in the
caller. We can let dwc3_prepare_one_trb() itself handle that part.

This simplifies the prototype of the function by removing two arguments.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>