usb: dwc3: better name for our request management lists

request_list and req_queued were, well, weird naming
choices.

Let's give those better names and call them,
respectively, pending_list and started_list. These
new names better reflect what these lists are
supposed to do.

While at that also rename req->queued to req->started.

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