usb: dwc3: gadget: use num_trbs when skipping TRBs on ->dequeue()

Now that we track how many TRBs a request uses, it's easier to skip
over them in case of a call to usb_ep_dequeue(). Let's do so and
simplify the code a bit.

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