usb: dwc3: gadget: start transfer on XFER_COMPLETE

if by the time we get to XFER_COMPLETE we have
pending requests to be processed, instead of waiting
for a following XFER_NOT_READY, let's start the request
right away and, maybe, save the time of a few NAKs
due to lack of started transfers.

Signed-off-by: Felipe Balbi <balbi@ti.com>