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

commit c3acd59014148470dc58519870fbc779785b4bf7 upstream

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.

Cc: Fei Yang <fei.yang@intel.com>
Cc: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: linux-usb@vger.kernel.org
Cc: stable@vger.kernel.org # 4.19.y
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
(cherry picked from commit c3acd59014148470dc58519870fbc779785b4bf7)
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>