usb: dwc3: gadget: always try to prepare on started_list first

In cases where we're given an SG-list which is
longer than the amount of currently available TRBs,
we will be left with the same request on
started_list and we should prioritize that request
over possible new requests on pending_list. That's
a way to guarantee requests complete in order.

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