Revert "usb: dwc3: gadget: start transfer on XFER_COMPLETE"

This reverts commit 8a1a9c9e
("usb: dwc3: gadget: start transfer on XFER_COMPLETE"). This
optimization is resulting into transfer stall for an endpoint.
Hence start transfer only from XFER_NOT_READY event.

Change-Id: Idcd7ae3de3cb85b84c3db7f367b56b097561b2dd
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>