usb: dwc3: gadget: increment dequeue pointer on completion

Instead of waiting until giveback before
incrementing the dequeue pointer, we can increment
it from dwc3_cleanup_done_reqs(), that way we avoid
an extra loop over all TRBs during giveback.

While at that, also avoid using req->first_trb_index
as that's completely unnecessary. A follow-up patch
will clean up further uses of that and remove the
field altogether.

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