usb: dwc3: gadget: offset Start Transfer latency for bulk EPs

usb: dwc3: gadget: offset Start Transfer latency for bulk EPs

We can offset the latency of a full Start Transfer
command - where we _must_ poll for its completion -
to usb_ep_enable() time. This means that once
requests start showing up from the gadget driver, we
can rely on No Response Update Transfer command -
where we don't need to poll for completion.

This patch, starts implementing this method for Bulk
endpoints, even though, technically, we could extend
it to all other endpoints in future commits.

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