usb: dwc3: gadget: extract dwc3_gadget_ep_skip_trbs()

commit 7746a8dfb3f9c91b3a0b63a1d5c2664410e6498d upstream

Extract the logic for skipping over TRBs to its own function. This
makes the code slightly more readable and makes it easier to move this
call to its final resting place as a following patch.

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 7746a8dfb3f9c91b3a0b63a1d5c2664410e6498d)
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>